gettext: fix legal info

The license is wrong, it's GPLv3+ rather than GPLv2+ now.
Also there's a specific clause for libintl that makes it LGPLv2.1+, see
gettext-runtime/COPYING.

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