Add lxml to pip-requires

Change-Id: I3f406ab0e2527c4c8e539b75432aa7d938daf73e
diff --git a/tools/pip-requires b/tools/pip-requires
index 2e1dc96..d3f9db7 100644
--- a/tools/pip-requires
+++ b/tools/pip-requires
@@ -3,3 +3,4 @@
 httplib2>=0.7.0
 pika
 unittest2
+lxml