preg_replace in where should replace all, not just first instance

Takes care of issues where posts still won't show up under **Mine** in the posts list if they are marked private and your co-author doesn't have `read_private_posts` capability already. This is because that type of query has a where clause in which `post_author` is compared more than just once. We should just replace all instances of the `post_author` comparison, not just the first.