block: fix CDROM dependency on BLK_DEV

After the cdrom cleanup, I get randconfig warnings for some configurations:

warning: (BLK_DEV_IDECD && BLK_DEV_SR) selects CDROM which has unmet direct dependencies (BLK_DEV)

This adds an explicit BLK_DEV dependency for both drivers. The other
drivers that select 'CDROM' already have this and don't need a change.

Fixes: 2a750166

 ("block: Rework drivers/cdrom/Makefile")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>