fix unused parameter warnings

Summary: Fix two small warnings when building AsyncUDPSocket on macOS.

Reviewed By: fanzeyi

Differential Revision: D20525411

fbshipit-source-id: 39cd84364413d434d261c17c4c8a87e31ac75a42

folly/io/async/AsyncUDPSocket.cpp

@@ -444,6 +444,8 @@ void AsyncUDPSocket::fillMsgVec(
       msg.msg_controllen = 0;
     }
 #else
+    (void)gso;
+    (void)gsoControl;
     msg.msg_control = nullptr;
     msg.msg_controllen = 0;
 #endif