Fixed avfmediaplayersession.mm for OSX 10.10 Yosemite.

Implicit cast caused build to fail. Explicit cast builds fine.

Task-number: QTBUG-41136
Change-Id: I0147c26a0f8b8198d6ed9944311547b81a50bccb
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>