db_rewrite_sql breaks queries using HAVING

HAVING is a DB construct which isn't often used, but is actually ANSI SQL. Unfortuntely, Drupal doesn't support it (read: It mutilates queries that use it) if one uses a node_access module (see http://drupal.org/node/157896).

The attached mini-patch solves the problem.

The patch is against D5 but also applies against D6-CVS (and even 4.7).