src: remove inclusion of iptables.h

iptables.h and ip6tables.h only include declarations internal to
iptables (specifically iptables.c and ip6tables.c), as most of the
public API has been moved to xtables.h a few months ago.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>
diff --git a/extensions/libip6t_icmp6.c b/extensions/libip6t_icmp6.c
index 2b3bd68..b87538f 100644
--- a/extensions/libip6t_icmp6.c
+++ b/extensions/libip6t_icmp6.c
@@ -4,7 +4,7 @@
 #include <string.h>
 #include <stdlib.h>
 #include <getopt.h>
-#include <ip6tables.h>
+#include <xtables.h>
 #include <linux/netfilter_ipv6/ip6_tables.h>
 
 struct icmpv6_names {