xz-utils: needs threads

It's one of the major features of the 5.2.0 release. Fixes:
http://autobuild.buildroot.net/results/e86/e86e97ff7887f1bdc6bf913498392ff50f56ea88/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>