Expose the dynamic cpu thread pool executor gflag define in header file

Summary: Trivial change to allow different services to turn this flag off at runtime

Reviewed By: djwatson

Differential Revision: D8060492

fbshipit-source-id: 8a62065ee1913fc67b6ae6983d03625849e139b3

folly/executors/CPUThreadPoolExecutor.h

@@ -19,6 +19,8 @@
 #include <folly/DefaultKeepAliveExecutor.h>
 #include <folly/executors/ThreadPoolExecutor.h>
 
+DECLARE_bool(dynamic_cputhreadpoolexecutor);
+
 namespace folly {
 
 /**