extensions: add missing limits.h include

Thanks to Stephen Hemminger for noticing.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
diff --git a/extensions/libip6t_icmp6.c b/extensions/libip6t_icmp6.c
index 69171d1..0678aac 100644
--- a/extensions/libip6t_icmp6.c
+++ b/extensions/libip6t_icmp6.c
@@ -5,6 +5,7 @@
 #include <stdlib.h>
 #include <getopt.h>
 #include <xtables.h>
+#include <limits.h> /* INT_MAX in ip6_tables.h */
 #include <linux/netfilter_ipv6/ip6_tables.h>
 
 struct icmpv6_names {