Fix invalid usage of meson objects which violates official meson
specification and thus breaks muon, an implementation of meson
written in C.
Reviewed-by: Mihai Preda <mhpreda@gmail.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15715>