LibJS: Correctly forward declare "Argument" as a struct

Libraries/LibJS/Forward.h

@@ -29,7 +29,6 @@
 namespace JS {
 
 class ASTNode;
-class Argument;
 class Cell;
 class Error;
 class Exception;
@@ -48,6 +47,8 @@ class Statement;
 class Value;
 enum class DeclarationType;
 
+struct Argument;
+
 template<class T>
 class Handle;