contrib: ffmpeg: fix availability #if test

It's available in macOS 10.13, not macOS 13.0 ¹. The __builtin_available() is correct but not the #if around it.

1. https://developer.apple.com/documentation/videotoolbox/vtcopysupportedpropertydictionaryforencoder(width:height:codectype:encoderspecification:encoderidout:supportedpropertiesout:) ↩