[NETFILTER]: nf_nat: autoload IPv4 connection tracking

Without this patch, the generic L3 tracker would kick in
if nf_conntrack_ipv4 was not loaded before nf_nat, which
would lead to translation problems with ICMP errors.

NAT does not make sense without IPv4 connection tracking
anyway, so just add a call to need_ipv4_conntrack().

Signed-off-by: Jan Engelhardt <jengelh@computergmbh.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>