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>