Fix building Metal backend with CMake

Change-Id: Ica940030367327cb2e35f90b1cc6b4202d4f3422
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/227062
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
1 file changed