libunwind: patch to silence warning

Not sure why this warning is being emitted; let's reexamine it on the
next libunwind upgrade. I triggered it with this:

zig c++ -o hello hello.cpp -target x86_64-windows

lib/libunwind/src/Unwind-seh.cpp

@@ -28,6 +28,8 @@
 #include "libunwind_ext.h"
 #include "UnwindCursor.hpp"
 
+#pragma clang diagnostic ignored "-Wdll-attribute-on-redeclaration"
+
 using namespace libunwind;
 
 #define STATUS_USER_DEFINED (1u << 29)

lib/libunwind/src/UnwindLevel1-gcc-ext.c

@@ -22,6 +22,8 @@
 #include "Unwind-EHABI.h"
 #include "unwind.h"
 
+#pragma clang diagnostic ignored "-Wdll-attribute-on-redeclaration"
+
 #if defined(_LIBUNWIND_BUILD_ZERO_COST_APIS)
 
 #if defined(_LIBUNWIND_SUPPORT_SEH_UNWIND)

lib/libunwind/src/UnwindLevel1.c

@@ -30,6 +30,8 @@
 #include "libunwind_ext.h"
 #include "unwind.h"
 
+#pragma clang diagnostic ignored "-Wdll-attribute-on-redeclaration"
+
 #if !defined(_LIBUNWIND_ARM_EHABI) && !defined(__USING_SJLJ_EXCEPTIONS__)
 
 #ifndef _LIBUNWIND_SUPPORT_SEH_UNWIND

lib/libunwind/src/libunwind.cpp

@@ -30,6 +30,8 @@
 #include "AddressSpace.hpp"
 #include "UnwindCursor.hpp"
 
+#pragma clang diagnostic ignored "-Wdll-attribute-on-redeclaration"
+
 using namespace libunwind;
 
 /// internal object to represent this processes address space