message: Fix Vulkan Swiftshader build on macOS
  There was a typo in macro name used to do platform detection.
diff: constexpr char kSwiftshaderLibName[] = "libvk_swiftshader.so";
  # elif defined(DAWN_PLATFORM_WINDOWS)
  constexpr char kSwiftshaderLibName[] = "vk_swiftshader.dll";
  -# elif defined(DAWN_PLATFORM_MAC)
  +# elif defined(DAWN_PLATFORM_MACOS)
  constexpr char kSwiftshaderLibName[] = "libvk_swiftshader.dylib";
  # else
  # error "Unimplemented Swiftshader Vulkan backend platform"
  