zstd: fix LICENSE statement

zstd: fix LICENSE statement

zstd is dual-licensed under BSD _OR_ GPLv2. License wording in the
README for v1.5.5 is misleading, but license headers in the code clearly
state that there is a choice between the two licenses.

Signed-off-by: Massimiliano Minella <massimiliano.minella@se.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 40f85de590c188c9c3985e64a83efaf06b0b4fbc)
Signed-off-by: Steve Sakoman <steve@sakoman.com>