lz4: fix legal info

It moved, and programs are GPLv2+. Fixes:
http://autobuild.buildroot.net/results/70a/70a526b265bb16a6be9f25c5044c65070706171c/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>