systemd: needs kernel headers >= 3.10

Systemd needs a toolchain with kernel headers >= 3.10, as it needs
IFLA_VLAN_PROTOCOL from linux/if_link.h.

Fixes: http://autobuild.buildroot.net/results/33e/33e1447949d13bccc4076b69f902432e2131cc1c



Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>