[FIX] common.py: missing import

It was not possible to call "about" function via remote control
(XML-RPC), because it contains string translation without import.

openerp/service/common.py

@@ -4,6 +4,7 @@ import logging
 
 import openerp.release
 import openerp.tools
+from openerp.tools.translate import _
 
 import security