multimedia/gstreamer1-libav: unbreak with ffmpeg 5.0

gstavdemux.c:486:26: error: no member named 'index_entries' in 'struct AVStream'

fftarget = stream->index_entries[keyframeidx].timestamp;
           ~~~~~~  ^

PR: 261302
Regressed by: https://git.ffmpeg.org/gitweb/ffmpeg.git/commitdiff/cea7c19cda0e