(websocket server) fix regression from 8.1.2, where per-deflate message compression was always disabled

ixwebsocket/IXWebSocket.h

@@ -57,6 +57,7 @@ namespace ix
         void setPingTimeout(int pingTimeoutSecs);
         void enablePong();
         void disablePong();
+        void enablePerMessageDeflate();
         void disablePerMessageDeflate();
         void addSubProtocol(const std::string& subProtocol);