compile fix

Change-Id: Ic854161a3104349656fbb3035daeb4adb4e79ba9
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>

src/plugins/coreplugin/mainwindow.cpp

@@ -109,6 +109,7 @@
 #include <QMessageBox>
 #include <QMenuBar>
 #include <QPushButton>
+#include <QStyleFactory>
 
 /*
 #ifdef Q_OS_UNIX