Fix C10_MOBILE macro for ios (#19779)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19779
This macro wasn't set correctly because the target macros weren't included from Apple's header.
Reviewed By: dzhulgakov
Differential Revision: D15090427
fbshipit-source-id: 43ca44f0f409e11718b7f60c3fdcd2aa02d7018e
