Inherit SocketServer directly from VNCServer

This makes VNCServer a sufficiently complete interface that callers
don't need to know about the specific implementation (VNCServerST
currently). And assuming that all servers will use sockets is not
that outrageous.
diff --git a/common/rfb/VNCServer.h b/common/rfb/VNCServer.h
index 5bb7c0b..298326f 100644
--- a/common/rfb/VNCServer.h
+++ b/common/rfb/VNCServer.h
@@ -22,15 +22,16 @@
 #ifndef __RFB_VNCSERVER_H__
 #define __RFB_VNCSERVER_H__
 
+#include <network/Socket.h>
+
 #include <rfb/UpdateTracker.h>
 #include <rfb/SSecurity.h>
 #include <rfb/ScreenSet.h>
 
-namespace network { class Socket; }
-
 namespace rfb {
 
-  class VNCServer : public UpdateTracker {
+  class VNCServer : public UpdateTracker,
+                    public network::SocketServer {
   public:
     // blockUpdates()/unblockUpdates() tells the server that the pixel buffer
     // is currently in flux and may not be accessed. The attributes of the