Make CPUThreadPoolExecutor::add(_, _, _, _) virtual [RFC]

Summary: I want to override this method in the next [diff](https://www.internalfb.com/diff/D31995497?entry_point=27), so I am making it virtual

Reviewed By: yfeldblum, jackerghan

Differential Revision: D31995385

fbshipit-source-id: 20855cfee0bcf809bdd477ff4820613180986178

folly/executors/CPUThreadPoolExecutor.h

@@ -134,7 +134,7 @@ class CPUThreadPoolExecutor : public ThreadPoolExecutor {
       Func expireCallback = nullptr) override;
 
   void addWithPriority(Func func, int8_t priority) override;
-  void add(
+  virtual void add(
       Func func,
       int8_t priority,
       std::chrono::milliseconds expiration,