headers_check fix: linux/ip6_tunnel.h

fix the following 'make headers_check' warning:

  include/linux/ip6_tunnel.h:21: found __[us]{8,16,32,64} type without #include <linux/types.h>
Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>

include/linux/ip6_tunnel.h

 #ifndef _IP6_TUNNEL_H
 #define _IP6_TUNNEL_H
 
+#include <linux/types.h>
+
 #define IPV6_TLV_TNL_ENCAP_LIMIT 4
 #define IPV6_DEFAULT_TNL_ENCAP_LIMIT 4