Mark pointer as FOLLY_NONNULL to satisfy nullability-completeness req…

…uirement

Summary:
I do not have context to know whether this is nullable or non-nullable but this allows me to build the target without getting an error about nullability-completeness.

```
./xplat/folly/executors/CPUThreadPoolExecutor.h:182:25: error: pointer is missing a nullability type specifier (_Nonnull, _Nullable, or _Null_unspecified) [-Werror,-Wnullability-completeness]
  182 |   BlockingQueue<CPUTask>* getTaskQueue();
      |                         ^
./xplat/folly/executors/CPUThreadPoolExecutor.h:182:25: note: insert '_Nullable' if the pointer may be null
```

Reviewed By: yfeldblum

Differential Revision: D63988501

fbshipit-source-id: 98e0c2864c96022cbcb4355ffa0cec79893a7776

folly/executors/CPUThreadPoolExecutor.h

@@ -179,7 +179,7 @@ class CPUThreadPoolExecutor : public ThreadPoolExecutor,
   static const size_t kDefaultMaxQueueSize;
 
  protected:
-  BlockingQueue<CPUTask>* getTaskQueue();
+  BlockingQueue<CPUTask>* FOLLY_NONNULL getTaskQueue();
   std::unique_ptr<ThreadIdWorkerProvider> threadIdCollector_{
       std::make_unique<ThreadIdWorkerProvider>()};