audio/forked-daapd: unbreak with ffmpeg 4.0

artwork.c:448:19: error: use of undeclared identifier 'CODEC_FLAG_GLOBAL_HEADER'

dst->flags |= CODEC_FLAG_GLOBAL_HEADER;
              ^

transcode.c:29:10: fatal error: 'libavfilter/avfiltergraph.h' file not found

^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

transcode.c:562:76: error: use of undeclared identifier 'CODEC_CAP_DELAY'

if (!(ctx->ofmt_ctx->streams[stream_index]->codec->codec->capabilities & CODEC_CAP_DELAY))
                                                                         ^

transcode.c:807:20: error: use of undeclared identifier 'CODEC_FLAG_GLOBAL_HEADER'

enc_ctx->flags |= CODEC_FLAG_GLOBAL_HEADER;
                  ^

PR: 227726
Reported by: antoine (via exp-run)