Define cork() as pure virtual in Socket class

This makes it possible to create a derived class from Socket which is
not TCP based, without having VNCSConnectionST.cxx trying to call
setsockopt() on a non-socket.
diff --git a/common/network/TcpSocket.h b/common/network/TcpSocket.h
index a97e683..c1b142f 100644
--- a/common/network/TcpSocket.h
+++ b/common/network/TcpSocket.h
@@ -62,9 +62,9 @@
     virtual bool sameMachine();
 
     virtual void shutdown();
+    virtual bool cork(bool enable);
 
     static bool enableNagles(int sock, bool enable);
-    static bool cork(int sock, bool enable);
     static bool isListening(int sock);
     static int getSockPort(int sock);
   private: