Changes to public.bzl to support building on iOS with objc_library bazel target.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1639443002

Review URL: https://codereview.chromium.org/1639443002
1 file changed