LibJS: Actually override is_private_identifier() in PrivateIdentifier

Regression from 72689ce.

Author: awesomekling

Userland/Libraries/LibJS/AST.h

@@ -1220,6 +1220,8 @@ class PrivateIdentifier final : public Expression {
     virtual Completion execute(Interpreter&, GlobalObject&) const override;
     virtual void dump(int indent) const override;
 
+    virtual bool is_private_identifier() const override { return true; }
+
 private:
     FlyString m_string;
 };