drm/virtio: fix up for include file changes

virtgpu_kms now uses VIRTIO_RING_F_INDIRECT_DESC, so it must
include virtio_ring.h directly.

Fixes: 5edbb560 ("drm/virtio: fix ring free check")
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Link: https://lore.kernel.org/r/20200407130542.0e3b5d9d@canb.auug.org.auSigned-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/gpu/drm/virtio/virtgpu_kms.c

@@ -25,6 +25,7 @@
 
 #include <linux/virtio.h>
 #include <linux/virtio_config.h>
+#include <linux/virtio_ring.h>
 
 #include <drm/drm_file.h>