CMake: Test tools require threads
CMake: Test tools require threads

If libiio was compiled without any backend enabled the build of the
tools would fail with undefined references to libpthread, at least on
Buildroot (strangely not on my Debian PC).

Signed-off-by: Paul Cercueil <73d292422978d45c9eeef455bb1b24a0db453722@analog.com>

analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio