Fix build warning with Qt >= 5.14.0

Qt 5.14 (late 2019) introduced new version 3 of QML type registration
structs. New version came with new struct fields, which cased compiler
to complain about missing field initializer for generalizedQobjectApi.

warning: src/qtdeclarative.rs: In function 'void __cpp_closure_9949451391349003668(const char* const&, const int&, const int&, const char* const&, const QMetaObject* const&, QObject* (* const&)(QQmlEngine*, QJSEngine*))':
warning: src/qtdeclarative.rs:548:9: warning: missing initializer for member 'QQmlPrivate::RegisterSingletonType::generalizedQobjectApi' [-Wmissing-field-initializers]
warning:   548 |         };
warning:       |         ^

By the way, they will be reset to 0 and rewritten completely in Qt 6.
See QTBUG-82241.

Author: ratijas

qmetaobject/src/qtdeclarative.rs

@@ -535,7 +535,11 @@ pub fn qml_register_singleton_type<T: QObject + QSingletonInit + Sized + Default
             /*instanceMetaObject*/ meta_object,
             // new in version 2
             /*typeId*/ ptrType,
-            /*revision*/ 0
+            /*revision*/ 0,
+#if QT_VERSION >= QT_VERSION_CHECK(5,14,0)
+            // new in version 3
+            /*generalizedQobjectApi*/ {}
+#endif
         };
 
         QQmlPrivate::qmlregister(QQmlPrivate::SingletonRegistration, &api);