Fix CMake build on macOS

Fixed: dawn:1061
Change-Id: Ie4208f32e9403220d78601ebefcc62c2c44c3485
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/62040
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>

src/common/CMakeLists.txt

@@ -69,6 +69,10 @@ if (WIN32)
         "WindowsUtils.h"
         "windows_with_undefs.h"
     )
+elseif(APPLE)
+    target_sources(dawn_common PRIVATE
+        "SystemUtils_mac.mm"
+    )
 endif()
 
 target_link_libraries(dawn_common PUBLIC dawncpp_headers PRIVATE dawn_internal_config)