virtio_blk: make serial attribute static

It's never declared so no need to make it extern.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>

drivers/block/virtio_blk.c

@@ -332,7 +332,8 @@ static ssize_t virtblk_serial_show(struct device *dev,
 
 	return err;
 }
-DEVICE_ATTR(serial, S_IRUGO, virtblk_serial_show, NULL);
+
+static DEVICE_ATTR(serial, S_IRUGO, virtblk_serial_show, NULL);
 
 static void virtblk_config_changed_work(struct work_struct *work)
 {