[gnome-2048/arnaudb/wip/gtk4: 30/36] Adapt to Overlay API.

[Arnaud B. <arnaudb src gnome org>]

commit e50b4f715182affe772cdac4a9d757b42ee4f2e9
Author: Arnaud Bonatti <arnaud bonatti gmail com>
Date:   Mon Jul 13 16:59:43 2020 +0200

    Adapt to Overlay API.

 src/game.vala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/game.vala b/src/game.vala
index dfd2122..4818cbe 100644
--- a/src/game.vala
+++ b/src/game.vala
@@ -104,7 +104,7 @@ private class Game : Gtk.Widget
         _foreground_grid.margin_bottom = 8;
 
         Gtk.Overlay overlay = new Gtk.Overlay ();
-        overlay.add (_background_grid);
+        overlay.set_child (_background_grid);
         overlay.add_overlay (_foreground_grid);
         overlay.insert_after (this, /* insert first */ null);