manifest: Explicitly tell polkit we want systemd
manifest: Explicitly tell polkit we want systemd

See https://bugs.freedesktop.org/show_bug.cgi?id=61127

GNOME/gnome-ostree,vrutkovs/vrutkovs.github.io,GNOME/gnome-ostree,chergert/gnome-continuous,vrutkovs/vrutkovs.github.io,chergert/gnome-continuous,chergert/gnome-continuous,GNOME/gnome-ostree,vrutkovs/vrutkovs.github.io