gpu-dawn: workaround missing pthread issue in abseil-cpp

abseil-cpp now depends on `pthread.h`, it previously didn't. But this appears to
be a bug in abseil of sorts, because if you have `ABSL_FORCE_THREAD_IDENTITY_MODE` set
to `ABSL_THREAD_IDENTITY_MODE_USE_CPP11` it doesn't appear to be used/needed.

[Zig doesn't ship with pthread for the MinGW / `x86_64-windows-gnu` target](https://github.com/ziglang/zig/issues/10989),
and so the header is missing - but we don't actually need it, so we just add an
empty file to prevent the missing include error.

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

gpu-dawn/build.zig

@@ -1130,6 +1130,7 @@ fn buildLibAbseilCpp(b: *Builder, step: *std.build.LibExeObjStep, options: Optio
         "-DD3D10_ARBITRARY_HEADER_ORDERING",
         "-D_CRT_SECURE_NO_WARNINGS",
         "-DNOMINMAX",
+        include("src/dawn/zig_mingw_pthread"),
     }) catch unreachable;
 
     // absl