add header guards

Change-Id: I3680ea31bf279cc6bde58c15a3dfa6b49815cf7e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/225138
Auto-Submit: Hal Canary <halcanary@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
2 files changed