audio/cmus: unbreak with ffmpeg 4.0

checking for FFMPEG_LIBS (pkg-config)... -L/usr/local/lib -lavformat -lavcodec
checking for FFMPEG_CFLAGS (pkg-config)... -I/usr/local/include
checking for header <libavcodec/avcodec.h>... yes
checking for successful build of ffmpeg.c... no
[...]
ip/ffmpeg.c:255:29: error: use of undeclared identifier 'CODEC_CAP_TRUNCATED'

if (codec->capabilities & CODEC_CAP_TRUNCATED)
                          ^

ip/ffmpeg.c:256:17: error: use of undeclared identifier 'CODEC_FLAG_TRUNCATED'

cc->flags |= CODEC_FLAG_TRUNCATED;
             ^

PR: 227726
Obtained from: upstream (via Greg V)