systemd: needs kernel headers >= 3.8

Systemd needs Linux headers >= 3.8 because it uses IFLA_GRE_FLOWINFO,
IFLA_IPTUN_FLAGS from linux/if_tunnel.h.

Fixes http://autobuild.buildroot.org/results/ad3/ad3b4003dc50582a493f1156234d83652104a6bf/



Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>