netfilter: fix IPv6 NAT dependencies in Kconfig
* NF_NAT_IPV6 requires IP6_NF_IPTABLES
* IP6_NF_TARGET_MASQUERADE, IP6_NF_TARGET_NETMAP, IP6_NF_TARGET_REDIRECT
and IP6_NF_TARGET_NPT require NF_NAT_IPV6.
This change just mirrors what IPv4 does in Kconfig, for consistency.
Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>