Add pyzfs BuildRequires for mock(1)

When building pyzfs under mock the python-cffi and python-setuptools
packages need to be installed and have been added to the BuildRequires.

Reviewed-by: Neal Gompa <ngompa@datto.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8265