LibGfx: SystemTheme is a struct, not a class

SystemTheme.h:81:1: error: 'SystemTheme' defined as a struct here but previously declared as a class; this is valid, but may result in linker errors under the Microsoft C++ ABI

Libraries/LibGfx/Forward.h

@@ -46,7 +46,7 @@ class Point;
 class Rect;
 class Size;
 class StylePainter;
-class SystemTheme;
+struct SystemTheme;
 class Triangle;
 
 enum class BitmapFormat;