python: fix build problems with Cython 0.20

python: fix build problems with Cython 0.20

Resolve the following build problem with Cython 0.20; I've also tested
this change on Cython 0.19.2 without problem.

>>>
Error compiling Cython file:
------------------------------------------------------------
...
        Helper function which should only be used internally by
        SyscallFilter objects and exists for the sole purpose of making it
        easier to deal with the varadic functions of the libseccomp API,
        e.g. seccomp_rule_add().
        """
        return self._arg
                  ^
------------------------------------------------------------

seccomp.pyx:194:19: Cannot convert 'scmp_arg_cmp' to Python object
>>>

Signed-off-by: Paul Moore <pmoore@redhat.com>