Binding example: Fix type registration

Fixes: QTBUG-81335
Change-Id: Ie93c9b7ad49703bce245592e659ccdb6ea2b7ed9
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

examples/qml/referenceexamples/binding/happybirthdaysong.h

@@ -52,6 +52,7 @@
 
 #include <QQmlPropertyValueSource>
 #include <QQmlProperty>
+#include <qqml.h>
 
 #include <QStringList>
 
@@ -60,6 +61,7 @@ class HappyBirthdaySong : public QObject, public QQmlPropertyValueSource
     Q_OBJECT
     Q_PROPERTY(QString name READ name WRITE setName NOTIFY nameChanged)
     Q_INTERFACES(QQmlPropertyValueSource)
+    QML_ELEMENT
 public:
     HappyBirthdaySong(QObject *parent = nullptr);