add license header to all std lib files

add SPDX license identifier
copyright ownership is zig contributors

lib/std/priority_queue.zig

@@ -1,3 +1,8 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2015-2020 Zig Contributors
+// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
+// The MIT license requires this copyright notice to be included in all copies
+// and substantial portions of the software.
 const std = @import("std.zig");
 const Allocator = std.mem.Allocator;
 const assert = std.debug.assert;