smack: needs kernel headers v3.0+

smack uses symbols that are exported to userspace only since Linux kernel
commit 1dbe39424a43 (xattr.h: expose string defines to userspace).

Fixes:
http://autobuild.buildroot.net/results/7fc/7fc69002fbb37962e65e0f83384ae7a4e6eac8a9/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>