softwarerenderer example: fix ambiguous reference to QQuickItem

GCC 6 says (with paths trimmed)
In file included from customrenderitem.cpp:47:0:
softwarerenderer.h:51:35: error: expected ')' before '*' token
softwarerenderer.h:61:5: error: reference to 'QQuickItem' is ambiguous

Change-Id: I70b328426ac48f5f6140d1ad4f015858979b99a5
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

examples/quick/scenegraph/rendernode/softwarerenderer.h

@@ -42,8 +42,7 @@
 #define SOFTWARERENDERER_H
 
 #include <qsgrendernode.h>
-
-class QQuickItem;
+#include <QQuickItem>
 
 class SoftwareRenderNode : public QSGRenderNode
 {