Add gif sources to public.bzl.

This also removes SK_HAS_GIF_LIBRARY since it does not appear in the
code.

Change-Id: I6e4d6ed560eb2877e6db61edf15f6be832166b44
Reviewed-on: https://skia-review.googlesource.com/3752
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
3 files changed