service: Port from g_type_class_add_private() to G_ADD_PRIVATE()
service: Port from g_type_class_add_private() to G_ADD_PRIVATE()

The former has been deprecated for a long time.

Signed-off-by: Philip Withnall <29c2a2ac2a854b9cf047275aa9de346d096be2e0@endlessm.com>

GNOME/dconf,GNOME/dconf,GNOME/dconf,GNOME/dconf