media-video/mpv: add LGPL-2.1+ to LICENSE in 9999

Upstream started a process of relicensing to LGPL-2.1+ a while ago
by collecting licensing agreements from developers.
Recently a bunch of files were relicensed from GPL-2+ to LGPL-2.1+.

Upstream-commit: 611426d162e182c2f863993d2b695ae6af2a5486
Upstream-commit: 803a0cebc46581cbef1f156fa65b4ad9ece65de7
Upstream-commit: 6b1374b203e8ada4bdb52d9e6367bfadcc9572c3

And so on.

Package-Manager: Portage-2.3.5, Repoman-2.3.2

media-video/mpv/mpv-9999.ebuild

@@ -25,7 +25,7 @@ SRC_URI+=" https://waf.io/waf-${WAF_PV}"
 DOCS+=( README.md )
 
 # See Copyright in sources and Gentoo bug 506946. Waf is BSD, libmpv is ISC.
-LICENSE="GPL-2+ BSD ISC"
+LICENSE="LGPL-2.1+ GPL-2+ BSD ISC"
 SLOT="0"
 IUSE="+alsa aqua archive bluray cdda +cli coreaudio cplugins cuda doc drm dvb
  dvd +egl encode gbm +iconv jack jpeg lcms +libass libav libcaca libmpv +lua