python-cffi: add missing runtime dependency
python-pycparser is mentioned in setup.py as install_requires, so
select it in Config.in.
As python-cffi will be installed with python-crossbar's dependencies,
remove it from python-crossbar's Config.in.
Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>