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/libs/utils/flowlayout.cpp

@@ -34,6 +34,7 @@
 
 #include <QtCore/QRect>
 #include <QtGui/QWidgetItem>
+#include <QtGui/QWidget>
 
 using namespace Utils;