systemd: fix license according to README file

Fixes:
  http://autobuild.buildroot.org/results/6e3/6e3dace5892a589225a93c896917833ebc8028f3



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>