[Metal] Add one missing label

Bug: skia:13446
Change-Id: I8c16561c62ea0defa408387c8a9b3dd0774c784e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/565637
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
2 files changed