Do not exclude Gif on iOS

We no longer depend on an external library, so there is no reason to
exclude it.
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2447973004

NOTREECHECKS=true

Review-Url: https://codereview.chromium.org/2447973004
diff --git a/public.bzl b/public.bzl
index 8636d29..7b16f38 100644
--- a/public.bzl
+++ b/public.bzl
@@ -250,7 +250,6 @@
         "src/utils/mac/*.cpp",
     ],
     exclude = [
-        "src/codec/*Gif*.cpp",
         "src/codec/*Ico*.cpp",
         "src/codec/*Jpeg*.cpp",
         "src/codec/*Webp*.cpp",