lavc/videotoolbox: Fix videotoolbox compile error on OS X 10.8.

Fixes error:
libavcodec/videotoolbox.c:511:18: error: implicit declaration of function
'CMVideoFormatDescriptionCreateFromH264ParameterSets' is invalid in C99

This was added in 10.9:
https://developer.apple.com/reference/coremedia/1489818-cmvideoformatdescriptioncreatefr?language=objc
