rsslisting example: don't leak the RSSListing object

It was new'd and unparented, so its destructor was never getting called.
A stack instance will do just fine.

Task-number: QTBUG-111228
Change-Id: If362b0e1e5b50d711a8fe8e01a8830034edf5363
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Dennis Oberst <dennis.oberst@qt.io>
(cherry picked from commit b1d59d6dd97e99086d15b29765282866500942f3)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

examples/corelib/serialization/rsslisting/main.cpp

@@ -20,7 +20,7 @@ Provides the main function for the RSS news reader example.
 int main(int argc, char **argv)
 {
     QApplication app(argc, argv);
-    RSSListing *rsslisting = new RSSListing;
-    rsslisting->show();
+    RSSListing rsslisting;
+    rsslisting.show();
     return app.exec();
 }