gstav: Use libavcodec util function for version check

The version of libavutil is printed in the log instead of libavcodec
because avutil_version() returns LIBAVUTIL_VERSION_INT. This can be confusing,
so we should be replace it with avcodec_version().
11 jobs for fix-avcodec-version in 34 minutes and 43 seconds (queued for 2 seconds)