virtio: include asm/barrier explicitly

virtio_ring.h uses mb() and friends, make
it pull in asm/barrier.h itself, not rely
on other headers to do it.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

include/linux/virtio_ring.h

 #ifndef _LINUX_VIRTIO_RING_H
 #define _LINUX_VIRTIO_RING_H
 
+#include <asm/barrier.h>
 #include <linux/irqreturn.h>
 #include <uapi/linux/virtio_ring.h>