Add missing include path for MacOS platforms

While integrating the project into our Bazel workspace, we had some
problems while building protoc in an Intel MacBook.

From my testing, the issue seems to be caused by missing include path
in the toolchain configuration to the `-none` variant of the headers.
More details below.

The issue can be reproduced directly inside `bazel-zig-cc` by including
the protobuf workspace as a dependency:

+http_archive(
+    name = "com_google_protobuf",
+    sha256 = "2d9084d3dd13b86ca2e811d2331f780eb86f6d7cb02b405426e3c80dcbfabf25",
+    strip_prefix = "protobuf-3.21.1",
+    url = "https://github.com/protocolbuffers/protobuf/archive/v3.21.1.zip",
+)
+
+load("@com_google_protobuf//:protobuf_deps.bzl", "protobuf_deps")
+
+protobuf_deps()

And then executing:

bazel build --platforms @zig_sdk//platform:darwin_amd64 @com_google_protobuf//:protoc

The command fails with the same error both in native compilation and
cross-compiling from Linux. The error:

ERROR: <output_base>/external/zlib/BUILD.bazel:37:11: Compiling compress.c failed: undeclared inclusion(s) in rule '@zlib//:zlib':
this rule is missing dependency declarations for the following files included by 'compress.c':
  '<output_base>/external/zig_sdk/lib/libc/include/x86_64-macos.10-none/sys/cdefs.h'
  '<output_base>/external/zig_sdk/lib/libc/include/x86_64-macos.10-none/sys/_symbol_aliasing.h'
  '<output_base>/external/zig_sdk/lib/libc/include/x86_64-macos.10-none/machine/limits.h'
  '<output_base>/external/zig_sdk/lib/libc/include/x86_64-macos.10-none/i386/limits.h'
  '<output_base>/external/zig_sdk/lib/libc/include/x86_64-macos.10-none/i386/_limits.h'
  '<output_base>/external/zig_sdk/lib/libc/include/x86_64-macos.10-none/sys/syslimits.h'
  '<output_base>/external/zig_sdk/lib/libc/include/x86_64-macos.10-none/machine/types.h'
  '<output_base>/external/zig_sdk/lib/libc/include/x86_64-macos.10-none/i386/types.h'
  '<output_base>/external/zig_sdk/lib/libc/include/x86_64-macos.10-none/i386/_types.h'
  '<output_base>/external/zig_sdk/lib/libc/include/x86_64-macos.10-none/sys/_types/_int8_t.h'
  '<output_base>/external/zig_sdk/lib/libc/include/x86_64-macos.10-none/sys/_types/_uintptr_t.h'
  '<output_base>/external/zig_sdk/lib/libc/include/x86_64-macos.10-none/machine/_types.h'
  '<output_base>/external/zig_sdk/lib/libc/include/x86_64-macos.10-none/sys/_pthread/_pthread_types.h'
  '<output_base>/external/zig_sdk/lib/libc/include/x86_64-macos.10-none/machine/endian.h'
  '<output_base>/external/zig_sdk/lib/libc/include/x86_64-macos.10-none/i386/endian.h'
  '<output_base>/external/zig_sdk/lib/libc/include/x86_64-macos.10-none/libkern/_OSByteOrder.h'
  '<output_base>/external/zig_sdk/lib/libc/include/x86_64-macos.10-none/libkern/i386/_OSByteOrder.h'
  '<output_base>/external/zig_sdk/lib/libc/include/x86_64-macos.10-none/sys/_types/_fd_def.h'
  '<output_base>/external/zig_sdk/lib/libc/include/x86_64-macos.10-none/Availability.h'
  '<output_base>/external/zig_sdk/lib/libc/include/x86_64-macos.10-none/AvailabilityInternal.h'
  '<output_base>/external/zig_sdk/lib/libc/include/x86_64-macos.10-none/sys/_pthread/_pthread_attr_t.h'
  '<output_base>/external/zig_sdk/lib/libc/include/x86_64-macos.10-none/sys/_pthread/_pthread_cond_t.h'
  '<output_base>/external/zig_sdk/lib/libc/include/x86_64-macos.10-none/sys/_pthread/_pthread_condattr_t.h'
  '<output_base>/external/zig_sdk/lib/libc/include/x86_64-macos.10-none/sys/_pthread/_pthread_rwlock_t.h'
  '<output_base>/external/zig_sdk/lib/libc/include/x86_64-macos.10-none/sys/_pthread/_pthread_rwlockattr_t.h'
  '<output_base>/external/zig_sdk/lib/libc/include/x86_64-macos.10-none/sys/_pthread/_pthread_t.h'
Target @com_google_protobuf//:protoc failed to build

(<output_base> replaced manually to keep the description sane)

Note that the specific rule included in the error message will vary due
to parallelism, but the error is always the same and the missing paths
always have the same prefix (up to `x86_64-macos.10-none`).

After applying the patch, we confirmed that both native and cross compilations
works as expected and can properly build protoc.

Signed-off-by: Luis Holanda <luiscmholanda@gmail.com>

toolchain/private/defs.bzl

@@ -57,6 +57,7 @@ def _target_darwin(gocpu, zigcpu):
             "libunwind/include",
             # TODO: Define a toolchain for each minimum OS version
             "libc/include/{}-macos.{}-gnu".format(zigcpu, min_os),
+            "libc/include/{}-macos.{}-none".format(zigcpu, min_os),
             "libc/include/any-macos.{}-any".format(min_os),
             "libc/include/any-macos-any",
         ],