fix tar command to backup log files

:Release Notes:
fix tar command to backup log files

:Detailed Notes:
previous_backup_logs.tar.gz is not recorded properly due to
"/var/log/" directory's attribute has changed.

:Testing Performed:
Compiled locally and tested on a device.

:QA Notes:

:Issues Addressed:
[GF-53809] backup log file is not stored properly

Open-webOS-DCO-1.0-Signed-off-by: Hongjeon Ha <hj.ha@lge.com>

Change-Id: I6510936cde91c73194b71ce31f3f658bbaf01b9a
Reviewed-on: https://g2g.palm.com/4103
Reviewed-by: DCO Verification
Reviewed-by: Hongjeon Ha <hj.ha@lge.com>
Tested-by: Hongjeon Ha <hj.ha@lge.com>
Reviewed-by: Gayathri Srinivasan <gayathri.srinivasan@lge.com>