python-json-schema-validator: needs versiontools on the host

python-json-schema-validator does not need versiontools on the target,
but only on the host, as it's listed in setup_requires in setup.py.

This was not noticed so far because host Python interpreter is started
with a PYTHONPATH that contains a directory with target Python
packages, so versiontools was found there. But as we are about to fix
PYTHONPATH to no longer include such a directory,
python-json-schema-validator would fail due to versiontools being
missed on the host.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>