target/loop: depend on SCSI

The target loopback driver is a low-level driver for the SCSI subsystem,
and as such needs to depend on it.

Fixes: 8a39a047

 ("target: don't depend on SCSI")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>