evtx: fix import error for lxml

Author: williballenthin

Evtx/Evtx.py

@@ -491,5 +491,5 @@ def lxml(self):
         Raises:
           ImportError: if lxml is not installed.
         '''
-        import lxml
+        import lxml.etree
         return lxml.etree.fromstring((e_views.XML_HEADER + self.xml()).encode('utf-8'))