Missing check for XPath

The config script is missing a check for the python module Xpath.

Traceback (most recent call last):
File "/usr/local/lib/opendnssec/signer/Engine.py", line 51, in <module>
import Util
File "/usr/local/lib/opendnssec/signer/Util.py", line 33, in <module>
from Ft.Xml.XPath import Evaluate
ImportError: No module named Ft.Xml.XPath