netfilter: avoid build error if TPROXY/SOCKET=y && NF_DEFRAG_IPV6=m

With TPROXY=y but DEFRAG_IPV6=m we get build failure:

net/built-in.o: In function `tproxy_tg_init':
net/netfilter/xt_TPROXY.c:588: undefined reference to `nf_defrag_ipv6_enable'

If DEFRAG_IPV6 is modular, TPROXY must be too.
(or both must be builtin).

This enforces =m for both.

Reported-and-tested-by: Liu Hua <[e-mail]>
Signed-off-by: Florian Westphal <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/netfilter/Kconfig

···
 	depends on NETFILTER_XTABLES
 	depends on NETFILTER_ADVANCED
 	depends on (IPV6 || IPV6=n)
+	depends on (IP6_NF_IPTABLES || IP6_NF_IPTABLES=n)
 	depends on IP_NF_MANGLE
 	select NF_DEFRAG_IPV4
 	select NF_DEFRAG_IPV6 if IP6_NF_IPTABLES
···
 	depends on NETFILTER_ADVANCED
 	depends on !NF_CONNTRACK || NF_CONNTRACK
 	depends on (IPV6 || IPV6=n)
+	depends on (IP6_NF_IPTABLES || IP6_NF_IPTABLES=n)
 	select NF_DEFRAG_IPV4
 	select NF_DEFRAG_IPV6 if IP6_NF_IPTABLES
 	help