extensions: add missing limits.h include

Thanks to Stephen Hemminger for noticing.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
diff --git a/extensions/libipt_DNAT.c b/extensions/libipt_DNAT.c
index b3a3557..b5f8028 100644
--- a/extensions/libipt_DNAT.c
+++ b/extensions/libipt_DNAT.c
@@ -6,6 +6,7 @@
 #include <getopt.h>
 #include <xtables.h>
 #include <iptables.h> /* get_kernel_version */
+#include <limits.h> /* INT_MAX in ip_tables.h */
 #include <linux/netfilter_ipv4/ip_tables.h>
 #include <net/netfilter/nf_nat.h>