Fix a hard-coded database table typo. s/wp_users/$wpdb->users/

git-svn-id: https://develop.svn.wordpress.org/trunk@15702 602fd350-edb4-49c9-b593-d223f7449a82

wp-includes/user.php

@@ -416,7 +416,7 @@ class WP_User_Query {
 				FROM wp_posts
 				$where
 				GROUP BY post_author
-			) p ON (wp_users.ID = p.post_author)
+			) p ON ({$wpdb->users}.ID = p.post_author)
 			";
 			$orderby = 'post_count';
 		}