Include synchronization/SaturatingSemaphore.h in the makefile

Summary:
It was missing, so it wasn't being installed.

Fixes https://github.com/facebook/folly/issues/727

Reviewed By: Orvid

Differential Revision: D6642322

fbshipit-source-id: 70c10e1265410f73b6c0cd759ce51a5176bfef83

folly/Makefile.am

@@ -438,6 +438,7 @@ nobase_follyinclude_HEADERS = \
 	synchronization/CallOnce.h \
 	synchronization/LifoSem.h \
 	synchronization/ParkingLot.h \
+	synchronization/SaturatingSemaphore.h \
 	synchronization/detail/AtomicUtils.h \
 	synchronization/detail/Sleeper.h \
 	system/MemoryMapping.h \