buildable: Make vfunc accessor functions private

buildable: Make vfunc accessor functions private

With the exception of gtk_buildable_get_id(), those are only used
to construct objects from XML descriptions, which is functionality
internal to GTK.

The API is therefore unlikely to be missed, and keeping it internal
means they can no longer unintentionally shadow object methods in
bindings with less namespacing; for example it's currently ambiguous
whether `infoBar.add_child()` refers to gtk_info_bar_add_child() or
gtk_buildable_add_child().

https://gitlab.gnome.org/GNOME/gtk/-/issues/3191