LibWeb: Stub out SVGMaskElement

Just enough that we stop creating layout nodes for mask elements, which
was making some SVG content look very wrong. :^)

Userland/Libraries/LibWeb/Forward.h

@@ -592,6 +592,7 @@ class SVGGeometryElement;
 class SVGGraphicsElement;
 class SVGLength;
 class SVGLineElement;
+class SVGMaskElement;
 class SVGPathElement;
 class SVGPolygonElement;
 class SVGPolylineElement;