Fix consistent crash on OS X when rendering to PDF format.

By implementing a dummy platform native interface (for now, it may be
extended in the near future), QPlatformPrinterSupportPlugin will avoid
the path of loading the printer support plugin
(QCocoaPrinterSupportPlugin on OS X).

Related upstream bug (QCocoaPrinterSupportPlugin problem with static build):
https://bugreports.qt-project.org/browse/QTBUG-33109

https://github.com/ariya/phantomjs/issues/12500