Header compatibility with Mac Catalyst.
Header compatibility with Mac Catalyst.

Summary:
In T97824645 a test that was failing for other reason was fixed and started
failing because it could not compile IGListKit.

The problem is that IGListKit was not prepared to be compiled for Mac Catalyst
since it was only testing for iOS platforms to include UIKit. These changes
modify the header to work in Mac Catalyst.

Differential Revision: D30410680

fbshipit-source-id: f886826b9c485acfb039a9e681afd85f9186c344

Instagram/IGListKit,Instagram/IGListKit,Instagram/IGListKit,Instagram/IGListKit,Instagram/IGListKit