drm/virtio: Make virtgpu_dmabuf_ops with static keyword

Fix the following sparse warning:

./virtgpu_prime.c:46:33: warning: symbol 'virtgpu_dmabuf_ops' was not declared. Should it be static?
Signed-off-by: NZou Wei <zou_wei@huawei.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1605338173-22100-1-git-send-email-zou_wei@huawei.comSigned-off-by: NGerd Hoffmann <kraxel@redhat.com>

drivers/gpu/drm/virtio/virtgpu_prime.c

@@ -43,7 +43,7 @@ static int virtgpu_virtio_get_uuid(struct dma_buf *buf,
 	return 0;
 }
 
-const struct virtio_dma_buf_ops virtgpu_dmabuf_ops =  {
+static const struct virtio_dma_buf_ops virtgpu_dmabuf_ops =  {
 	.ops = {
 		.cache_sgt_mapping = true,
 		.attach = virtio_dma_buf_attach,