Apply the OS_CODE macro redefinition workaround to iOS builds

The issue described in https://flutter.googlesource.com/third_party/freetype2/+/af4c2d86d399b15f31f19aafe49e540578a6be39
happens in both macOS targets and iOS targets when building with the latest
version of Clang.

Change-Id: Ia7684ba11d0ac36074b1361fa1374ee8a59b86f0
Reviewed-on: https://flutter-review.googlesource.com/c/third_party/freetype2/+/57540
Reviewed-by: Zach Anderson <zra@google.com>