Extension only API flag
Extension only API flag

Summary:
Adds `APPLICATION_EXTENSION_API_ONLY = YES` to Xcode project.
Closes https://github.com/Instagram/IGListKit/pull/79

Differential Revision: D4028543

Pulled By: rnystrom

fbshipit-source-id: 203afacb5485dd4dc5897991d7981cf80f6da993

Instagram/IGListKit,Instagram/IGListKit,Instagram/IGListKit,zhubofei/IGListKit,zhubofei/IGListKit,zhubofei/IGListKit,Sherlouk/IGListKit,zhubofei/IGListKit,Sherlouk/IGListKit,charles0126dev/IGListKit,dshahidehpour/IGListKit,dshahidehpour/IGListKit,Instagram/IGListKit,Sherlouk/IGListKit,charles0126dev/IGListKit,Instagram/IGListKit,dshahidehpour/IGListKit,zhubofei/IGListKit,charles0126dev/IGListKit