package/libvpx: bump version to 1.15.0
Release notes:
https://chromium.googlesource.com/webm/libvpx/+/refs/tags/v1.15.0
https://chromium.googlesource.com/webm/libvpx/+/refs/tags/v1.14.1
https://chromium.googlesource.com/webm/libvpx/+/refs/tags/v1.14.0
Signed-off-by: Bernd Kuhls <bernd@kuhls.net>
Signed-off-by: Julien Olivain <ju.o@free.fr>