Fix iptables --modprobe parameter (Maurice van der Pot <griffon26@kfk4ever.com>)

Supply modprobe parameter to iptables_insmod function.

Bugzilla #556
diff --git a/iptables.c b/iptables.c
index dee695f..60ddc2d 100644
--- a/iptables.c
+++ b/iptables.c
@@ -1861,7 +1861,7 @@
 	static int ret = -1;
 
 	if (!loaded) {
-		ret = iptables_insmod("ip_tables", NULL, quiet);
+		ret = iptables_insmod("ip_tables", modprobe, quiet);
 		loaded = (ret == 0);
 	}