package/tvheadend: requires kernel headers >= 3.2

tvheadend requires SYS_TURBO, introduced in linux 3.2.

Fixes:
    http://autobuild.buildroot.net/results/4df/4df8cd85e0287910567df81c0394b2914570e98a



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