LibWeb: Add CSSNamespaceRule to Forward.h

Author: https://github.com/AtkinsSJ
Commit: 36e3963263
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3464
Reviewed-by: https://github.com/ADKaster

Libraries/LibWeb/Forward.h

@@ -149,6 +149,7 @@ class CSSKeywordValue;
 class CSSLayerBlockRule;
 class CSSLayerStatementRule;
 class CSSMediaRule;
+class CSSNamespaceRule;
 class CSSNestedDeclarations;
 class CSSOKLab;
 class CSSOKLCH;