Update the copyright year

The constant was neglected when updating all copyright years.

include/constants.php

@@ -25,7 +25,7 @@ define('SM_VERSION', '1.5.2 [SVN]');
  * Year interval for copyright notices in the interface
  * @since 1.5.2
  */
-define('SM_COPYRIGHT', '1999-2009');
+define('SM_COPYRIGHT', '1999-2012');
 
 /**************************************************************/
 /* Set values for constants used by SquirrelMail preferences. */