BugFix: QueuedImmediateExecutor.h is missed in the include header files

Summary:
As per title.
Closes https://github.com/facebook/folly/pull/827

Reviewed By: Orvid

Differential Revision: D7721855

Pulled By: yfeldblum

fbshipit-source-id: fe335dcfa83b4db04616e33e353fea52ebb2bbf2

folly/Makefile.am

@@ -110,6 +110,7 @@ nobase_follyinclude_HEADERS = \
 	executors/IOObjectCache.h \
 	executors/IOThreadPoolExecutor.h \
 	executors/NotificationQueueExecutor.h \
+	executors/QueuedImmediateExecutor.h \
 	executors/ScheduledExecutor.h \
 	executors/SequencedExecutor.h \
 	executors/SerialExecutor.h \