Use of deprecated glib function 'g_type_class_add_private'

[PureTryOut]

As of commit 7e5db31d36532274c2b2428ef9bace796928785e of Glib, g_type_class_add_private has been deprecated. This currently prevents building.

hd-config-file.c: In function 'hd_config_file_class_init':
hd-config-file.c:261:3: error: 'g_type_class_add_private' is deprecated [-Werror=deprecated-declarations]
   g_type_class_add_private (g_object_class, sizeof (HDConfigFilePrivate));
   ^~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/glib-2.0/gobject/gobject.h:24,
                 from /usr/include/glib-2.0/gobject/gbinding.h:29,
                 from /usr/include/glib-2.0/glib-object.h:23,
                 from hd-config-file.c:28:
/usr/include/glib-2.0/gobject/gtype.h:1303:10: note: declared here
 void     g_type_class_add_private       (gpointer                    g_class,
          ^~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors