[SCSI] fix function prototype warning

int_to_scsilun() takes a pointer to a struct scsi_lun in it's
prototype, so add this structure to scsi_device.h to avoid declaration
inside function prototype warnings.
Signed-off-by: NJames Bottomley <James.Bottomley@SteelEye.com>

include/scsi/scsi_device.h

@@ -9,7 +9,7 @@
 struct request_queue;
 struct scsi_cmnd;
 struct scsi_mode_data;
-
+struct scsi_lun;
 
 /*
  * sdev state: If you alter this, you also need to alter scsi_sysfs.c