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 <mie@igel.co.jp>
Message-Id: <20230417022037.917668-1-mie@igel.co.jp>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>