kernel: add disabled CONFIG_IP6_NF_TARGET_MASQUERADE

kernel: add disabled CONFIG_IP6_NF_TARGET_MASQUERADE

In kernel commit adf82accc5f5 ("netfilter: x_tables: merge ip and
ipv6 masquerade modules") the config symbols IP_NF_TARGET_MASQUERADE
and IP6_NF_TARGET_MASQUERADE have been demoted to simple backwards-
compat options for NETFILTER_XT_TARGET_MASQUERADE.

In netfilter.mk, this has already been updated in OpenWrt commit
d1592306cc07 ("netfilter.mk: use CONFIG_NETFILTER_XT_TARGET_MASQUERADE"),
having us use the new config symbol.

However, enabling IP_NF_NAT or IP6_NF_NAT still makes the relevant
legacy options selectable, so we need to disable them in generic
config (and forget about them afterwards).
Since CONFIG_IP_NF_TARGET_MASQUERADE is already present there, this
just adds the missing CONFIG_IP6_NF_TARGET_MASQUERADE.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>