add missing import to mpls test

Change-Id: If53a7c8066ec9713645a009218e264384afe93e4
Signed-off-by: Gabriel Ganne <gabriel.ganne@qosmos.com>
diff --git a/test/test_mpls.py b/test/test_mpls.py
index 08cd55b..92a579f 100644
--- a/test/test_mpls.py
+++ b/test/test_mpls.py
@@ -1,6 +1,7 @@
 #!/usr/bin/env python
 
 import unittest
+import socket
 
 from framework import VppTestCase, VppTestRunner
 from vpp_sub_interface import VppSubInterface, VppDot1QSubint, VppDot1ADSubint