zstd: fix LICENSE statement

[Backport from OE-Core: 40f85de590c188c9c3985e64a83efaf06b0b4fbc]

zstd is dual-licensed under BSD _OR_ GPLv2 which was updated in the README for v1.5.6.
License wording in the README for v1.5.2 is misleading, but license headers in the code
clearly state that there is a choice between the two licenses.

(From OE-Core rev: 4ef9b4d947b6e8ed7637cdd9e722c3f0c83990dd)

Signed-off-by: Massimiliano Minella <massimiliano.minella@se.com>
Signed-off-by: Shubham Kulkarni <skulkarni@mvista.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>