Add utils/WGPUHelpers.{cpp,h} to CMake build

These files were added in this recent commit:
https://dawn-review.googlesource.com/c/dawn/+/68282

Change-Id: I63a64cea1946b3fe48ac20d478f9b7664bc63de0
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/68560
Commit-Queue: James Price <jrprice@google.com>
Commit-Queue: Austin Eng <enga@chromium.org>
Auto-Submit: James Price <jrprice@google.com>
Reviewed-by: Austin Eng <enga@chromium.org>

src/dawn_native/CMakeLists.txt

@@ -176,6 +176,8 @@ target_sources(dawn_native PRIVATE
     "VertexFormat.cpp"
     "VertexFormat.h"
     "dawn_platform.h"
+    "utils/WGPUHelpers.cpp"
+    "utils/WGPUHelpers.h"
 )
 target_link_libraries(dawn_native
     PUBLIC dawncpp_headers