gnu: meld: Wrap with Glib or Gtk path variables.

Fixes the example given at <https://issues.guix.gnu.org/issue/37942>.

* gnu/packages/gnome.scm (meld)[inputs]: Add gsettings-desktop-schemas.
[arguments]: Add (guix build glib-or-gtk-build-system) to #:modules
and #:imported-modules.  Use its ‘glib-or-gtk-wrap’ phase.