loxi-prep: remove uses of cstruct module

This module does not exist in PyLoxi.
diff --git a/tests/bsn_ipmask.py b/tests/bsn_ipmask.py
index 8304958..b26fe3c 100644
--- a/tests/bsn_ipmask.py
+++ b/tests/bsn_ipmask.py
@@ -6,7 +6,7 @@
 
 from oftest import config
 import oftest.controller as controller
-import oftest.cstruct as ofp
+import ofp
 import oftest.message as message
 import oftest.base_tests as base_tests