Partial compile fix for current Qt dev and/or Qt 6

This does not cover Utils::MapReduceOption with QVector, and code
that's not compiled on my machine.

Change-Id: Ib63923985c52b1bb74e5ec2068a2bb37469ac618
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/help/textbrowserhelpviewer.h

@@ -92,7 +92,11 @@ public:
     void scaleUp();
     void scaleDown();
 
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
     void setSource(const QUrl &name) override;
+#else
+    void doSetSource(const QUrl &url, QTextDocument::ResourceType type) override;
+#endif
 
     void withFixedTopPosition(const std::function<void()> &action);