stream_lavf: change license to LGPL

All authors agreed.

The author of 1ee8ce75 did not respond, but it was a mpv pull request,
and at this time DOCS/contribute.md and the "Copyright" file stated that
all contributions must include LGPL relicensing permission. But you
could claim that this was too "hidden". Sort of a corner case, I guess,
but not my problem.