Tracing: Make TimelineModel explicitly uncreatable

Otherwise the QML engine complains about it being uncreatable.

Change-Id: I955d123f8fc08a0a7350d3c86f3c524f784325ed
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/libs/tracing/timelinemodel.h

@@ -33,6 +33,7 @@ class TRACING_EXPORT TimelineModel : public QObject
     Q_PROPERTY(int count READ count NOTIFY contentChanged FINAL)
     Q_PROPERTY(int defaultRowHeight READ defaultRowHeight CONSTANT FINAL)
     QML_ELEMENT
+    QML_UNCREATABLE("")
 
 public:
     class TimelineModelPrivate;