webos-preferred-providers: Add webos-shutdownscripts to initscripts

:Release Notes:
Add webos-shutdownscripts to what's currently in
VIRTUAL-RUNTIME_initscripts, because if another layers replaces them
with something else, webos-shutdownscript will need to be replaced as
well.

:Detailed Notes:

:Testing Performed:
Checked rootfs content.

:QA Notes:

:Issues Addressed:
[GF-5716] Clean up rootfs and duplicate files

Open-webOS-DCO-1.0-Signed-off-by: Iyad Qumei <iyad.qumei@lge.com>

Change-Id: I2796c3cb0a5c984e54886086007db407747de89e
Reviewed-on: https://g2g.palm.com/3922
Reviewed-by: DCO Verification
Reviewed-by: Susan Montooth <susan.montooth@lge.com>
Tested-by: Susan Montooth <susan.montooth@lge.com>
Build: Susan Montooth <susan.montooth@lge.com>
Reviewed-by: Build Verification
Reviewed-by: Herb Kuta <herb.kuta@lge.com>
Reviewed-by: Jongho Kim <jongho1208.kim@lge.com>
Tested-by: Jongho Kim <jongho1208.kim@lge.com>