package/w_scan: requires kernel headers >= 3.0

It needs DVB API version 5.3 or later, introduced in linux 3.0.

Fixes:
    http://autobuild.buildroot.net/results/6e7/6e7c691099a8f3eef4fc84860ceb1a94f25873eb



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