libs: gui: Do not error on C linkage return type

Summary:
We need to support a user specified
return type with C linkage for compatibility shims.
This is not being set in our wrapper, because in most
cases, this is fixable (and should be fixed).

Test Plan: Only merge when we're building with clang

Reviewers: randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D597