netfilter: fix compilation of masquerading without IP_NF_TARGET_MASQUERADE

 CONFIG_NF_NAT_MASQUERADE_IPV6=m
 # CONFIG_IP6_NF_TARGET_MASQUERADE is not set

results in:

net/ipv6/netfilter/nf_nat_masquerade_ipv6.c: In function ‘nf_nat_masquerade_ipv6’:
net/ipv6/netfilter/nf_nat_masquerade_ipv6.c:41:14: error: ‘struct nf_conn_nat’ has no member named ‘masq_index’
  nfct_nat(ct)->masq_index = out->ifindex;
              ^
net/ipv6/netfilter/nf_nat_masquerade_ipv6.c: In function ‘device_cmp’:
net/ipv6/netfilter/nf_nat_masquerade_ipv6.c:61:12: error: ‘const struct nf_conn_nat’ has no member named ‘masq_index’
  return nat->masq_index == (int)(long)ifindex;
            ^
net/ipv6/netfilter/nf_nat_masquerade_ipv6.c:62:1: warning: control
reaches end of non-void function [-Wreturn-type]
 }
 ^
make[3]: *** [net/ipv6/netfilter/nf_nat_masquerade_ipv6.o] Error 1

Fix this by using the new NF_NAT_MASQUERADE_IPV4 and _IPV6 symbols
in include/net/netfilter/nf_nat.h.

Reported-by: Jim Davis <jim.epost@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>