Fix fibers build - Semaphore::future_wait declared but undefined
Fix fibers build - Semaphore::future_wait declared but undefined

Summary:
[Folly] Fix fibers build - `Semaphore::future_wait` declared but undefined.

The `#if` guarding related code is overly aggressive.

Reviewed By: Orvid

Differential Revision: D14498252

fbshipit-source-id: 4183fc67fece0e2bdad1fab5c83e7b9a50ae847b

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