systemd: fix license info

File is LICENSE.GPL2, not LICENSE.GPLV2, fixes:
http://autobuild.buildroot.net/results/e6c/e6c4dc22c014e62ce35273945a2a2307315ec3f7/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>