... and G_TYPE_INSTANCE_GET_PRIVATE()

g_type_class_add_private() and G_TYPE_INSTANCE_GET_PRIVATE() were
deprecated in GLib 2.58.  Instead, use
G_DEFINE_TYPE_WITH_PRIVATE(), and G_ADD_PRIVATE(), and the
implictly-defined foo_get_instance_private() functions, all of
which are available in the GLib version we depend on.