veth: add kernel header linux/veth.h for VETH defines

Similar to what's done with <linux/if_link.h>, make sure used defines
actually exist.  Otherwise building on even slightly older kernels
fails.

Taken from upstream kernel commit 1860e379875dfe7271c649058aeddffe5afd9d0d
(tag: v3.15), file 'include/uapi/linux/veth.h'.

Signed-off-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: Thomas Haller <thaller@redhat.com>