Add a missing MAYBE_UNUSED
Add a missing MAYBE_UNUSED

Summary:
Suppress warning when building without coroutines

(Note: this ignores all push blocking failures!)

Reviewed By: Mizuchi

Differential Revision: D35083113

fbshipit-source-id: 28f5b2565632edddec38b5a69b17739b464ab4c2

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