[tcpip] Allow supported address families to be detected at runtime

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/ipv6.c

@@ -1001,6 +1001,7 @@ struct tcpip_net_protocol ipv6_tcpip_protocol __tcpip_net_protocol = {
 	.name = "IPv6",
 	.sa_family = AF_INET6,
 	.header_len = sizeof ( struct ipv6_header ),
+	.net_protocol = &ipv6_protocol,
 	.tx = ipv6_tx,
 	.netdev = ipv6_netdev,
 };