lavfi/f_ebur128: relicense to LGPL

All copyright holders have agreed to the relicensing.

Approved-by: Andreas Cadhalpun <andreas.cadhalpun@googlemail.com>
Approved-by: David Sedacca <sedacca@comcast.net>
Approved-by: Ganesh Ajjanagadde <gajjanag@mit.edu>
Approved-by: Jean First <jeanfirst@gmail.com>
Approved-by: Kyle Swanson <k@ylo.ph>
Approved-by: Michael Niedermayer <michael@niedermayer.cc>
Approved-by: Nicolas George <george@nsup.org>
Approved-by: Paul B Mahol <onemda@gmail.com>
Approved-by: Thilo Borgmann <thilo.borgmann@mail.de>