Mod_speling wouldn't compile any more after the recent change to

use construct_url() because http_core.h wasn't included.
Include it to get construct_url()'s prototype.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@80104 13f79535-47bb-0310-9956-ffa450edef68