kernel: netfilter: Adapt merge ipv4/ipv6 masquerade code

kernel: netfilter: Adapt merge ipv4/ipv6 masquerade code

In kernel commit 0168e8b361 ("netfilter: nat: merge ipv4/ipv6 masquerade
code into main nat module") the CONFIG_NF_NAT_MASQUERADE_IPV4 and
CONFIG_NF_NAT_MASQUERADE_IPV6 kernel configuration option were changed
to bool and the code will not be compiled as a own module any more, but
it will be integrated into nf_nat_ipv4.ko or nf_nat_ipv6.ko to save some
memory.

Activate these options as bool in the generic kernel 4.19 configuration
only, to always build them into the nf_nat_ipv*.ko modules. The kmod
file will still try to select them as module, but the generic
configuration will not be overwritten.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>