Renamed CodecID to AVCodecID in modules/highgui/src/cap_ffmpeg_impl.hpp

The most recent version of ffmpeg doesn't have CodecID -- they're now using AVCodecID instead (for details, see https://github.com/FFmpeg/FFmpeg/commit/36ef5369ee9b336febc2c270f8718cec4476cb85 in their git repo).  This breaks the OpenCV build with the most recent version of ffmpeg.

Replacing CodecID with AVCodecID solves the problem.  

It's likely that in the future, they'll also do the same thing for CODEC_ID constants.  I left them alone, for now, since it still works.