packagegroup-agl-graphical-weston: Add polkit to RDEPENDS

packagegroup-agl-graphical-weston: Add polkit to RDEPENDS

Images that require/inherit packagegroup-agl-graphical-weston would pull
in polkit which is needed by weston/AGL compositor in order to be able
to switch VTs as a regular user without needing CAP_SYS_ADMIN
privileges.

Added initially a workaround, by incorrectly assuming that this was due
to missing psplash. Upon further investigation noticed that polkit
wasn't really installed which ultimately failed to activate the
session.

With this change, I'm able to boot-up and display the gallery app for
agl-image-flutter and cluster-dashboard application for the
agl-cluster-demo-platform image.

Bug-AGL: SPEC-4359, SPEC-4375

Suggested-by: Scott Murray <scott.murray@konsulko.com>
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: Ifd01fb61fc4d0f797519b54bf22395970e2a9fec
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/27482
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>