veth: use kernel header file

Use santized kernel header for veth.h and put in correct place
to prevent possible future problems with API.
Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>

ip/veth.h → include/net/veth.h

-#ifndef __NET_VETH_H__
-#define __NET_VETH_H__
+#ifndef __NET_VETH_H_
+#define __NET_VETH_H_
 
 enum {
 	VETH_INFO_UNSPEC,

ip/link_veth.c

@@ -11,12 +11,11 @@
  */
 
 #include <string.h>
+#include <net/if.h>
+#include <net/veth.h>
 
 #include "utils.h"
 #include "ip_common.h"
-#include "veth.h"
-
-#define	IFNAMSIZ	16
 
 static void usage(void)
 {