gnutls: Unified package names to lower-case

create-spdx can't detect the license properly if the case doesn't
match, so fix it.

Signed-off-by: Keiya Nobuta <nobuta.keiya@fujitsu.com>
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>