scsi: move function declarations to scsi_priv.h

We get 2 warnings about global functions without a declaration in the
scsi driver when building with W=1:

drivers/scsi/scsi_lib.c:467:6: warning: no previous prototype for 'scsi_requeue_run_queue' [-Wmissing-prototypes]
drivers/scsi/scsi_lib.c:2609:6: warning: no previous prototype for 'scsi_evt_thread' [-Wmissing-prototypes]

In fact, both functions are declared in drivers/scsi/scsi_scan.c but
need to move them into scsi_priv.h.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>