libiio: Fix pthread linking for iiod

libiio: Fix pthread linking for iiod

Update the iiod CMake configuration to resolve pthread linking issues
introduced with commit:

17ee15da70d23c314a3264c87a785101cbd9d85d
("libiio: Bump version to 0.6")

Fixes:
http://autobuild.buildroot.net/results/ffe/ffec14aa79833a6f0889a87b31dae464d083e6a8/
http://autobuild.buildroot.net/results/49f/49f249632ed021ebe73ee7fab1b55fb3bb4c08f8/

Signed-off-by: Matthew Fornero <mfornero@mathworks.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>