netfilter: tproxy: fix build with IP6_NF_IPTABLES=n

after commit 93742cf (netfilter: tproxy: remove nf_tproxy_core.h)

CONFIG_IPV6=y
CONFIG_IP6_NF_IPTABLES=n

gives us:

net/netfilter/xt_TPROXY.c: In function 'nf_tproxy_get_sock_v6':
net/netfilter/xt_TPROXY.c:178:4: error: implicit declaration of function 'inet6_lookup_listener'

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>