odd logout case..

odd logout case..
(in a tabbed browser, pull up SM, then click 'Sign Out' as an "open in new tab"
then click something in the current window, you'll get an error trying to
execute unknown function do_hook on line 88 of display_messages).

So we'll just add the require, just to make sure..

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@6718 7612ce4b-ef26-0410-bec9-ea0150e637f0