Fix tab extension dropdown having dead non-clickable hover area

Closes ppy#28899.

osu.Game/Graphics/UserInterface/OsuTabDropdown.cs

@@ -123,7 +123,7 @@ public OsuTabDropdownHeader()
                     }
                 };
 
-                Padding = new MarginPadding { Left = 5, Right = 5 };
+                Margin = new MarginPadding { Left = 5, Right = 5 };
             }
 
             protected override bool OnHover(HoverEvent e)