add include guard

Change-Id: I377e575433a0e35e2bc1856efebbfbfb6c6c5875
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/793837
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Mike Reed <mike@reedtribe.org>
1 file changed