extensions: add missing limits.h include

Thanks to Stephen Hemminger for noticing.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
diff --git a/xtables.c b/xtables.c
index f305673..1214127 100644
--- a/xtables.c
+++ b/xtables.c
@@ -32,6 +32,7 @@
 #include <arpa/inet.h>
 
 #include <xtables.h>
+#include <limits.h> /* INT_MAX in ip_tables.h/ip6_tables.h */
 #include <linux/netfilter_ipv4/ip_tables.h>
 #include <linux/netfilter_ipv6/ip6_tables.h>
 #include <libiptc/libxtc.h>