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>