Do not build SkGifCodec when building SkWuffsCodec

Bug: skia:8235
Bug: https://github.com/flutter/flutter/issues/23838

Make SkGifCodec + SkGifImageReader an optional component that is not
built when skia_use_wuffs is true.

Change-Id: Ief488ec6ed929e62f73b51b3a2e993d704d2493c
Reviewed-on: https://skia-review.googlesource.com/c/187782
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
1 file changed