Fix build failure on systems missing IPV6_TCLASS

Some operating systems such as GNU/Hurd support IPv6 but do not define
the IPV6_TCLASS socket option.

Signed-off-by: Mike Miller <mtmiller@debian.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>

ssl.c

@@ -913,8 +913,10 @@ int udp_sockaddr(struct openconnect_info *vpninfo, int port)
 	} else if (vpninfo->peer_addr->sa_family == AF_INET6) {
 		struct sockaddr_in6 *sin = (void *)vpninfo->dtls_addr;
 		sin->sin6_port = htons(port);
+#if defined(IPV6_TCLASS)
 		vpninfo->dtls_tos_proto = IPPROTO_IPV6;
 		vpninfo->dtls_tos_optname = IPV6_TCLASS;
+#endif
 	} else {
 		vpn_progress(vpninfo, PRG_ERR,
 			     _("Unknown protocol family %d. Cannot create UDP server address\n"),