add SkpFilePlayer to externs

Bug: skia:
Change-Id: I190b6ac004f68f699cab2ec38b7af25e5113e335
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/206680
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Nathaniel Nifong <nifong@google.com>
1 file changed