netfilter: Fix build errors with xt_socket.c

As reported by Randy Dunlap:

====================
when CONFIG_IPV6=m
and CONFIG_NETFILTER_XT_MATCH_SOCKET=y:

net/built-in.o: In function `socket_mt6_v1_v2':
xt_socket.c:(.text+0x51b55): undefined reference to `udp6_lib_lookup'
net/built-in.o: In function `socket_mt_init':
xt_socket.c:(.init.text+0x1ef8): undefined reference to `nf_defrag_ipv6_enable'
====================

Like several other modules under net/netfilter/ we have to
have a dependency "IPV6 disabled or set compatibly with this
module" clause.
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netfilter/Kconfig

@@ -1175,6 +1175,7 @@ config NETFILTER_XT_MATCH_SOCKET
 	depends on NETFILTER_XTABLES
 	depends on NETFILTER_ADVANCED
 	depends on !NF_CONNTRACK || NF_CONNTRACK
+	depends on (IPV6 || IPV6=n)
 	select NF_DEFRAG_IPV4
 	select NF_DEFRAG_IPV6 if IP6_NF_IPTABLES
 	help