buildroot

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>