Fix compilation with -no-pch

Q_UNUSED(cursor) was failing to compile when configured with
-no-pch.

Change-Id: I1da3c95c1636ca06f38a97052ee4360232520a8b
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

src/gui/kernel/qplatformnativeinterface.cpp

@@ -38,6 +38,7 @@
 ****************************************************************************/
 
 #include "qplatformnativeinterface.h"
+#include <QtGui/qcursor.h>
 
 QT_BEGIN_NAMESPACE