fix minor annoyance on z/OS: __FILE__ is set to

"./foo.c" instead of simply "foo.c", so filter
out all but the basename before logging it with
debug messages


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@178299 13f79535-47bb-0310-9956-ffa450edef68