sox: dep on ffmpeg, not libav

The libav recipe no longer exists, in favor of ffmpeg.

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Signed-off-by: Sujith Haridasan <Sujith_Haridasan@mentor.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Armin Kuster <akuster808@gmail.com>