[NETFILTER] IPV6: Fix dependencies.

Although the menu dependencies in net/ipv6/netfilter/Kconfig
guard the entries in that file from the Kconfig GUI, this does
not prevent them from being selected still via "make oldconfig"
when IPV6 etc. is disabled.

So add explicit dependencies.

Signed-off-by: David S. Miller <davem@davemloft.net>