[NETFILTER]: nf_nat: fix NF_NAT dependency

NF_NAT depends on NF_CONNTRACK_IPV4, not NF_CONNTRACK.

Signed-off-by: Yasuyuki Kozakai <yasuyuki.kozakai@toshiba.co.jp>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>