Do not depend on deprecated dbus-glib library

We don't actually use it here anyway. We also don't check for it, so
the fact that we could successfully #include it in the past was pure
coincidence (presumably one of our dependencies used to provide it).

Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: André Magalhães <andre.magalhaes@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D5495