fix build errors related to AVFoundation @ OS X <10.7

AVFoundation framework is available in OS X v10.7 and later

related commits:
  7822b5535cccc68e5b454d04a66a6b4f27675ec6
  0b49cc5b03b39aa1dfddf924c6eaf8cb42e791a8

Change-Id: If771fca79c41d70e51eab533faec7bb9997803b7
Reviewed-on: https://gerrit.libreoffice.org/9769
Reviewed-by: Tor Lillqvist <tml@collabora.com>
Tested-by: Tor Lillqvist <tml@collabora.com>