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 <liusdu@126.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>