netfilter: ipv6: Fix undefined symbol nf_ct_frag6_gather

CONFIG_NETFILTER=m and CONFIG_NF_DEFRAG_IPV6 is not set

ERROR: "nf_ct_frag6_gather" [net/ipv6/ipv6.ko] undefined!

Fixes: c9bb6165

 ("netfilter: nf_conntrack_bridge: fix CONFIG_IPV6=y")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: wenxu <wenxu@ucloud.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>