[gtkmm] Gtk: add missing conversion for experssions

[Andreas Persson <andreasp src gnome org>]

commit 2e8f88563b8656fe7c320a3f8461a07b142eae69
Author: Andreas Persson <andreasp56 outlook com>
Date:   Sun Sep 20 20:24:18 2020 +0200

    Gtk: add missing conversion for experssions
    
    I forgot one change in the previous get_expression commit: an m4
    conversion for const expressions.

 tools/m4/convert_gtk.m4 | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/tools/m4/convert_gtk.m4 b/tools/m4/convert_gtk.m4
index 7dccd88c..0218e224 100644
--- a/tools/m4/convert_gtk.m4
+++ b/tools/m4/convert_gtk.m4
@@ -607,3 +607,4 @@ _CONVERSION(`GtkFilter*',`Glib::RefPtr<Filter>',`Glib::wrap($3)')
 
 _CONVERSION(`const Glib::RefPtr<Expression<Glib::ustring>>&',`GtkExpression*',`(($3) ? ($3)->gobj() : 
nullptr)')
 _CONVERSION(`GtkExpression*',`Glib::RefPtr<Expression<Glib::ustring>>',`Glib::wrap<Glib::ustring>($3)')
+_CONVERSION(`GtkExpression*',`Glib::RefPtr<const Expression<Glib::ustring>>',`Glib::wrap<Glib::ustring>($3)')