Make AsyncUDPSocket::getGSO virtual

Summary: otherwise the one in Mock doesn't work

Reviewed By: yfeldblum

Differential Revision: D21264567

fbshipit-source-id: c73e6bac1f0293edb67355ee88b6c2ec1b9684fd

folly/io/async/AsyncUDPSocket.h

@@ -348,7 +348,7 @@ class AsyncUDPSocket : public EventHandler {
 
   // generic segmentation offload get/set
   // negative return value means GSO is not available
-  int getGSO();
+  virtual int getGSO();
 
   bool setGSO(int val);