Maildir: fix imports of emailutil

Commit 0c17350e4f revisited utime_for_header without taking care of the
imports.

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

offlineimap/folder/Base.py

@@ -19,7 +19,7 @@ import os.path
 import re
 from sys import exc_info
 
-from offlineimap import threadutil, emailutil
+from offlineimap import threadutil
 from offlineimap import globals
 from offlineimap.ui import getglobalui
 from offlineimap.error import OfflineImapError