viocd: needs to depend on BLOCK

Fix this build error when CONFIG_BLOCK is not set:
drivers/cdrom/viocd.c: In function 'viocd_blk_open':
drivers/cdrom/viocd.c:156: error: dereferencing pointer to incomplete type
drivers/cdrom/viocd.c: In function 'viocd_blk_release':
drivers/cdrom/viocd.c:162: error: dereferencing pointer to incomplete type
drivers/cdrom/viocd.c: In function 'viocd_blk_ioctl':
drivers/cdrom/viocd.c:170: error: dereferencing pointer to incomplete type
drivers/cdrom/viocd.c: In function 'viocd_blk_media_changed':
drivers/cdrom/viocd.c:176: error: dereferencing pointer to incomplete type
...

Signed-off-by: Alexander Beregalov <a.beregalov@gmail.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>