scsi: target: Don't request modules that aren't even built

If, for example, I don't enable CONFIG_TCM_PSCSI, then every time I load
the target subsystem, I get an annoying

    Unable to load target_core_pscsi

kernel log message.  Instead let's only request_module() on things if
that code is enabled.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>