Blocking fix for 1004831: transition: ffmpeg

[Peter Belkner <info@pbelkner.de>]

Dear Sebastian,

I'm referring to https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1004794.

You've reported a bunch of compiling errors.  AFAIK these are due to 
FFmpeg reorganizing their header files. Nowadays its required to 
explicitly include "avcodec/libavcodec.h" whereas in previous versions 
it was sufficient to include "avformat/libavformat.h" (which in turn 
included "avcodec/libavcodec.h").

Please note that bs1770gain-0.8.4.tar.gz follows the requirement changed 
by FFmpeg out of the blue.

Best regards

Peter