[Qt][Mac] Disable QTKit video on OS X.

https://bugs.webkit.org/show_bug.cgi?id=117591

Reviewed by Tor Arne Vestbø.

Source/WebCore:

Deployment of the same Qt binaries to different
Mac OS X version makes it impossible to link against
the correct WebKitSystemInterface version at build time.

No new tests, no behavioural change.

* WebCore.pri:

Tools:

Deployment of Qt binaries to different Mac OS X
platform version makes it impossible to link against
the correct version of the WebKitSystemInterface library,
so we need to disable QTKit for media elements.

* qmake/mkspecs/features/features.prf:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@151546 268f45cc-cd09-0410-ab3c-d52691b4dbfc
Change-Id: I8ad054ffc1896de1cc461efed0a5786337c29261
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>