[graphite] list MtlGraphiteTypesPriv.h in bazel/gn sources

Change-Id: Ie4565b23ed5db5b891de696a99ea568f4eccfda8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/946876
Reviewed-by: Kaylee Lubick <kjlubick@google.com>
Auto-Submit: Brian Salomon <briansalomon@gmail.com>
Commit-Queue: Nicolette Prevost <nicolettep@google.com>
Commit-Queue: Brian Salomon <briansalomon@gmail.com>
Reviewed-by: Nicolette Prevost <nicolettep@google.com>
2 files changed