libdrm: needs threads

libdrm select libpthread-stubs that needs threads support.

Fixes:
http://autobuild.buildroot.net/results/1da/1dad13110312a44b40b9ee4050b265de1f3c8a42/build-end.log



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