Fix build flags in GrMtlCaps::supportedReadPixelsColorType

TBR: egdaniel@google.com
Change-Id: I53618e650ae8ffd891d335a834990a4ec18efc72
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/229923
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
1 file changed