scsi: sd: add a SPDX tag to sd.c

sd.c is the only sd file missing licensing information.  Add a
GPLv2 tag for the default kernel license.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/sd.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  *      sd.c Copyright (C) 1992 Drew Eckhardt
  *           Copyright (C) 1993, 1994, 1995, 1999 Eric Youngdale