netfilter: xt_socket: missing ifdef CONFIG_IP6_NF_IPTABLES dependency

nf_defrag_ipv6_disable() requires CONFIG_IP6_NF_IPTABLES.

Fixes: 75063c92

 ("netfilter: xt_socket: fix a typo in socket_mt_destroy()")
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Eric <Dumazet&lt;edumazet@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>