audio/pianobar: unbreak with ffmpeg 4.0

In file included from src/main.c:53:
In file included from src/main.h:30:
src/player.h:37:10: fatal error: 'libavfilter/avfiltergraph.h' file not found
#include <libavfilter/avfiltergraph.h>

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

PR: 227726
Obtained from: upstream (via Homebrew)