Instead of using extra modules for these, turn the config options into
an implicit dependency that adds masq feature to the protocol specific nf_nat module.

before:
   text    data     bss     dec     hex filename
   2001     860       4    2865     b31 net/ipv4/netfilter/nf_nat_masquerade_ipv4.ko
   5579     780       2    6361    18d9 net/ipv4/netfilter/nf_nat_ipv4.ko
   2860     836       8    3704     e78 net/ipv6/netfilter/nf_nat_masquerade_ipv6.ko
   6648     780       2    7430    1d06 net/ipv6/netfilter/nf_nat_ipv6.ko

after:
   text    data     bss     dec     hex filename
   7245     872       8    8125    1fbd net/ipv4/netfilter/nf_nat_ipv4.ko
   9165     848      12   10025    2729 net/ipv6/netfilter/nf_nat_ipv6.ko
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>