mwan3: fix variable naming

The variable IPT is not valid at this point. Set the variable usage to IPT4
fixes this issue.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>

net/mwan3/files/lib/mwan3/mwan3.sh

@@ -1004,7 +1004,7 @@ mwan3_set_user_iptables_rule()
 			case $proto in
 				tcp|udp)
 				[ "$global_logging" = "1" ] && [ "$rule_logging" = "1" ] && {
-					$IPT -A mwan3_rules \
+					$IPT4 -A mwan3_rules \
 						-p $proto \
 						-s $src_ip \
 						-d $dest_ip $ipset \