Fix QtCompositor compilation

This is a quick fix, a proper fix needs the ongoing effort to make
surfaces and outputs aware of each other.

Change-Id: I61faca9929a448208e82574c5c4d963c271f7e68
Reviewed-by: Pier Luigi Fiorini <pierluigi.fiorini@gmail.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>

src/compositor/compositor_api/qwaylandquickoutput.cpp

@@ -71,7 +71,7 @@ void QWaylandQuickOutput::update()
 void QWaylandQuickOutput::updateStarted()
 {
     m_updateScheduled = false;
-    frameStarted();
+    compositor()->frameStarted();
     compositor()->cleanupGraphicsResources();
 }