libiio: test programs require threads

Fixes:
http://autobuild.buildroot.net/results/228/228a32167d05337acf17b0e3689aa91e8a67d8fb/
http://autobuild.buildroot.net/results/86a/86adb968659fec56684ba80e1c4230230fd7a1c8/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>