Add necessary variable

osu.Game/Overlays/BeatmapSet/Header.cs

@@ -30,6 +30,7 @@ public class Header : BeatmapDownloadTrackingComposite
         private const float buttons_spacing = 5;
 
         private readonly UpdateableBeatmapSetCover cover;
+        private readonly Box coverGradient;
         private readonly OsuSpriteText title, artist;
         private readonly AuthorInfo author;
         private readonly FillFlowContainer downloadButtonsContainer;