netfilter: Kconfig: Change select IPv6 dependencies

... from IPV6 to NF_TABLES_IPV6 and IP6_NF_IPTABLES.

In some cases module selects depend on IPV6, but this means that they
select another module even if eg. NF_TABLES_IPV6 is not set in which
case the selected module is useless due to the lack of IPv6 nf_tables
functionality.

The same applies for IP6_NF_IPTABLES and iptables.

Joint work with: Arnd Bermann <arnd@arndb.de>

Signed-off-by: Máté Eckl <ecklm94@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>