Use decltype(event::ev_fd) instead of #ifdef

Summary:
Use decltype(event::ev_fd) instead of #ifdef

(Note: this ignores all push blocking failures!)

Reviewed By: yfeldblum, kevin-vigor

Differential Revision: D18525100

fbshipit-source-id: 7cef32def7b40c71b2298f9dc354fbe63607fb8e

folly/portability/Event.h

@@ -31,9 +31,5 @@
 #include <folly/net/detail/SocketFileDescriptorMap.h>
 
 namespace folly {
-#ifdef _MSC_VER
-using libevent_fd_t = evutil_socket_t;
-#else
-using libevent_fd_t = int;
-#endif
+using libevent_fd_t = decltype(event::ev_fd);
 } // namespace folly