gvfs: drop dbus-glib dependency

Drop the dbus-glib dependency as gvfs uses dbus directly.

Also change dependency on the XML parser for dbus to be more
in sync with the dbus package.

[Peter: drop expat|libxml2 deps as shared-mime-type already selects libxml2]
Signed-off-by: Sven Neumann <s.neumann@raumfeld.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>