package/weston: requires kernel headers >= 3.0

It needs K_OFF, introduced in 2.6.39.

Since we do not have dependencies on kernel headers before 3.0,
just depend on kernel headers 3.0.

Fixes:
    http://autobuild.buildroot.net/results/aa5/aa54b1aaf0ac89531d7a1e7dd3900b35605ae3f5



Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>