QmlPreview: Fix missing include for older Qt

Otherwise the meta type system complains.
Amends bbed189656.

Change-Id: I7cd4a2b244d7b3920029a47a706be89735f67cd4
Reviewed-by: hjk <hjk@qt.io>

src/plugins/qmlpreview/qmlpreviewplugin.h

@@ -7,12 +7,11 @@
 #include "qmldebugtranslationclient.h"
 
 #include <extensionsystem/iplugin.h>
+#include <projectexplorer/runcontrol.h>
 #include <qmljs/qmljsdialect.h>
 
 namespace Core { class IEditor; }
 
-namespace ProjectExplorer { class RunControl; }
-
 namespace QmlDebug { class QmlDebugConnection; }
 
 namespace QmlPreview {