mesa3d: needs NPTL threading

Strictly speaking, linuxthreads (new) should be enough, since it includes
pthread_barrier_* implementation. However, we currently have no clean way to
distinguish between LT and LT.old.

Fixes:
http://autobuild.buildroot.net/results/66a/66a9d2c664e0a9b2b0c51b998c72c2ceda90ff9b/



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