Fix free mod selection sub-button being clickable even if the main button isn't

Noticed in passing when testing https://github.com/ppy/osu/pull/32674.

osu.Game/Screens/OnlinePlay/FooterButtonFreeMods.cs

@@ -80,6 +80,7 @@ namespace osu.Game.Screens.OnlinePlay
                     Origin = Anchor.Centre,
                     Scale = new Vector2(0.8f),
                     Icon = FontAwesome.Solid.Bars,
+                    Enabled = { BindTarget = Enabled },
                     Action = () => freeModSelectOverlay.ToggleVisibility()
                 }
             });