imaplibutil: add missing errno import

Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/imaplibutil.py

@@ -22,6 +22,7 @@ from sys import exc_info
 import threading
 from hashlib import sha1
 import socket
+import errno
 
 from offlineimap.ui import getglobalui
 from offlineimap import OfflineImapError