Remove warning about g_type_class_add_private being deprecated
When compiling GNOME Notes, there are a few warning in libgd about g_type_class_add_private being deprecated.
These are the objects that have that function:
`$ git grep g_type_class_add_private
libgd/gd-main-list-view.c: g_type_class_add_private (klass, sizeof (GdMainListViewPrivate));
libgd/gd-margin-container.c: g_type_class_add_private (klass, sizeof (GdMarginContainerPrivate));
libgd/gd-notification.c: g_type_class_add_private (object_class, sizeof (GdNotificationPrivate));
libgd/gd-tagged-entry.c: g_type_class_add_private (klass, sizeof (GdTaggedEntryPrivate));
libgd/gd-tagged-entry.c: g_type_class_add_private (klass, sizeof (GdTaggedEntryTagPrivate));`
Edited by Isaque Galdino de Araujo