scsi: libiscsi: Annotate fall-through

This patch avoids that building with W=1 causes the compiler to
complain about fall-through.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Lee Duncan <lduncan@suse.com>
Cc: Chris Leech <cleech@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>