vala: updated to 0.56.9

Vala 0.56.9
===========
* Various improvements and bug fixes:
  - codegen: Add missing EXTERN flag for GType function of error domains
  - codegen: Improve handling of sealed classes
  - vala: Report an error when trying to chain up to inaccessible private
    ctor
  - vala: Fix spaces for attributes when writing property accessors
  - girwriter: Fix c:type attribute for real struct parameters
  - testrunner: Add support to check generated header
  - manual: Update from wiki.gnome.org

* Bindings:
  - gio-2.0: Ease implemention of GLib.ActionGroup
  - glib-2.0: Fix return-type of GLib.SourceOnceFunc
  - glib-2.0: Add ConvertError.{NO_MEMORY,EMBEDDED_NUL}
  - gtk4: Pick up fix for Gdk.Toplevel.compute_size.size parameter
  - vapi: Add documentation to GLib.Math that '-X -lm' may be needed when
    compiling

(adam)