fix mac build
fix mac build

Summary: no fibers support

Reviewed By: zhxchen17

Differential Revision: D19950271

fbshipit-source-id: 4371102d95c2b7eb61803ca73035a64d6edf0dcc

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift