Fix a build with Qt 6

In Qt 6 implicit conversion between QFuture and other types
is forbidden. Make it explicit instead.
See ff0ba7e2d7b91fd5809cb314935a1ca1a436f6c9.

Change-Id: Ie42e6b9b5047ba5eeec9f63fd03179e73f95314d
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/qmljs/qmljsmodelmanagerinterface.h

@@ -183,6 +183,9 @@ public:
 
     // Blocks until all parsing threads are done. Use for testing only!
     void test_joinAllThreads();
+
+    template <typename T>
+    void addFuture(const QFuture<T> &future) { addFuture(QFuture<void>(future)); }
     void addFuture(const QFuture<void> &future);
 
     QmlJS::Document::Ptr ensuredGetDocumentForPath(const QString &filePath);