package/gstreamer1/gst1-libav: fix license

package/gstreamer1/gst1-libav: fix license

Commit 15dc48ca9becafec6ba11b08af18453bc0e40c9e forgot to change
the license which is LGPL-2.0+ since version 1.17.1 and
https://gitlab.freedesktop.org/gstreamer/gst-libav/-/commit/c437e175ad9c82732ac46e36ccc398ad041bf13b

It should be noted that even if the commit states LGPL-2.1+, the license
is LGPL-2.0+:
https://gitlab.freedesktop.org/gstreamer/gst-libav/-/issues/83

Fixes:
 - http://autobuild.buildroot.org/results/848d7c49a01f719397cb2e8a8f8e3bd73bd03df4

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>