Add include guard to SkFontConfigInterface.h

BUG=skia:7137

Change-Id: I29536a21211eae8b075d43984f3677f64ff9f481
Reviewed-on: https://skia-review.googlesource.com/57820
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
1 file changed