Utils/Coreplugin: Clean headers.

Change-Id: I1e0d432127651622d40ff990e1866eac9fac46a0
Reviewed-on: http://codereview.qt.nokia.com/3091
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>

src/plugins/coreplugin/mainwindow.cpp

@@ -72,6 +72,7 @@
 #include "statusbarwidget.h"
 #include "basefilewizard.h"
 #include "ioutputpane.h"
+#include "externaltoolmanager.h"
 #include "editormanager/systemeditor.h"
 
 #include <coreplugin/findplaceholder.h>