[skottie] Add missing source to public.bzl

https://skia-review.googlesource.com/c/skia/+/559097 introduced a new
compilation unit (BlendModes.cpp).

Change-Id: I8af1f089b33381dd7a412f34976034e189aa0bf7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/559376
Reviewed-by: Herb Derby <herb@google.com>
Auto-Submit: Florin Malita <fmalita@chromium.org>
Commit-Queue: Herb Derby <herb@google.com>
1 file changed