Silence warning about unknown warning
Silence warning about unknown warning

Summary:
Clang doesn't implement `-Wmaybe-uninitialized` so attempts to suppress it
generate `-Wunknown-warning-option`.  This diff changes the suppression of
`-Wmaybe-uninitialized` to be GCC-only to allow Clang to compile
`folly::unsafe_default_initialized_cv::operator T()`.

Reviewed By: smeenai

Differential Revision: D41203291

fbshipit-source-id: 51f12d7f480b7d785bd59ca51ac724656ab36706

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly