Don't build qwindowsurface autotest on Mac OS.

Don't build qwindowsurface autotest on Mac OS.

The test was a no-op on Mac OS. It's better to avoid building or running
it on Mac OS, rather than spending time compiling and running a test
that does nothing on that platform.

Change-Id: I4aa579036788e404f8a6a8f1a80acd32338fb3cd
Reviewed-on: http://codereview.qt-project.org/5958
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Rohan McGovern <rohan.mcgovern@nokia.com>