virtio_ring: add a struct device forward declaration

The virtio_ring header file uses the struct device without a forward
declaration.

Signed-off-by: Shunsuke Mie <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Michael S. Tsirkin <[e-mail]>

include/linux/virtio_ring.h

···
 
 struct virtio_device;
 struct virtqueue;
+struct device;
 
 /*
  * Creates a virtqueue and allocates the descriptor ring.  If