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

folly/fibers/Semaphore.cpp

@@ -116,6 +116,8 @@ coro::Task<void> Semaphore::co_wait() {
       std::memory_order_acquire));
 }
 
+#endif
+
 SemiFuture<Unit> Semaphore::future_wait() {
   auto oldVal = tokens_.load(std::memory_order_acquire);
   do {
@@ -136,8 +138,6 @@ SemiFuture<Unit> Semaphore::future_wait() {
   return makeSemiFuture();
 }
 
-#endif
-
 size_t Semaphore::getCapacity() const {
   return capacity_;
 }