Fix iOS builders.

Gyp... sigh.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4787

Change-Id: I5488bbc6a8e267dc42d1c716cf0f879b662af171
Reviewed-on: https://skia-review.googlesource.com/4787
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>