if_tunnel.h: add missing ams/byteorder.h include

When compiling userspace application which includes
if_tunnel.h and uses GRE_* defines you will get undefined
reference to __cpu_to_be16.

Fix this by adding missing #include <asm/byteorder.h>

Cc: stable@kernel.org
Signed-off-by: Paulius Zaleckas <paulius.zaleckas@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/if_tunnel.h

@@ -2,6 +2,7 @@
 #define _IF_TUNNEL_H_
 
 #include <linux/types.h>
+#include <asm/byteorder.h>
 
 #ifdef __KERNEL__
 #include <linux/ip.h>