x265: needs threads

Fixes [1]:

  build/x265-1.8/source/common/threading.h:34:21: fatal error: pthread.h: No such file or directory

[1] http://autobuild.buildroot.net/results/261/26134a234be5a67914cde0fb81601f6860d943d2



Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>