LibGfx: Forward declare Gfx::FontMetrics as a struct

Userland/Libraries/LibGfx/Forward.h

@@ -14,9 +14,9 @@ class Color;
 class DisjointRectSet;
 class Emoji;
 class Font;
-class FontMetrics;
 class GlyphBitmap;
 class ImageDecoder;
+struct FontMetrics;
 
 template<typename T>
 class Line;