netfilter: xt_TPROXY: undefined reference to `udp6_lib_lookup'

CONFIG_IPV6=m
CONFIG_NETFILTER_XT_TARGET_TPROXY=y

   net/built-in.o: In function `nf_tproxy_get_sock_v6.constprop.11':
>> xt_TPROXY.c:(.text+0x583a1): undefined reference to `udp6_lib_lookup'
   net/built-in.o: In function `tproxy_tg_init':
>> xt_TPROXY.c:(.init.text+0x1dc3): undefined reference to `nf_defrag_ipv6_enable'

This fix is similar to 1a5bbfc3 ("netfilter: Fix build errors with
xt_socket.c").

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