python-cryptography's build depends (host cffi, libffi) were transferred
to python-cffi at some point; this corrects the situation.

python-cryptography's host Python build depends is copied from its
setup.py[1].

[1]: https://github.com/pyca/cryptography/blob/2.6.1/setup.py#L47

Signed-off-by: Jeffery To <jeffery.to@gmail.com>