systemd: needs kernel headers >= 3.0

Systemd requires at least Linux headers 3.0 because it uses:

- setns(2) syscall, introduced in 3.0.
- IFLA_GROUP from if_link.h, introduced in 2.6.39.

Fixes http://autobuild.buildroot.net/results/098/09866da023d527b3c228a98f1fde104e780e1883/



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