Fix build failure wrt IP QoS support in libvncclient.

This is a small addendum to 0797e42a4a.
Seems that having IPv6 support in the OS does not necessarily mean that
IPV6_TCLASS is available. One such case seems to be Mac OS X 10.5.

libvncclient/sockets.c

@@ -599,7 +599,7 @@ SetDSCP(int sock, int dscp)
 
   switch(addr.sa_family)
     {
-#ifdef LIBVNCSERVER_IPv6
+#if defined LIBVNCSERVER_IPv6 && defined IPV6_TCLASS
     case AF_INET6:
       level = IPPROTO_IPV6;
       cmd = IPV6_TCLASS;