Fix clang issue on NotificationQueue::size

Summary: The implicit conversion loses integer precision: 'size_type' (aka 'unsigned
long') to 'int'.

Reviewed By: mzlee

Differential Revision: D2585883

fb-gh-sync-id: 1fc7c84b66c8f19cc36b798dd198730764e19b28

folly/io/async/NotificationQueue.h

@@ -419,7 +419,7 @@ class NotificationQueue {
     return true;
   }
 
-  int size() {
+  size_t size() {
     folly::SpinLockGuard g(spinlock_);
     return queue_.size();
   }