fix up ios.xsan builds

These used to work with custom/Homebrew Clang,
but I can only get them to link using Apple's now.

Change-Id: Ifa574207c693d147e074feb4166a8468e9402385
Reviewed-on: https://skia-review.googlesource.com/c/193543
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed