scsi_dh_alua: Declare local functions static

This patch avoids that building with W=1 causes gcc to report the
following type of warning:

    no previous prototype for ... [-Wmissing-prototypes]

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Hannes Reinicke <hare@suse.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Ewan Milne <emilne@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>