Fix building with FFmpeg 0.8.

avformat_find_stream_info was introduced in Libav avformat 53.3,
but it was only merged to FFmpeg in avformat 53.6.

In Libav avformat 53.3-53.5 av_find_stream_info is not removed
(only deprecated), so this shouldn't break building with that.
Merge pull request #1799 from SpecLad:avformat-53-6