use ap_snprintf() instead of snprintf()

this fixes a build error on OS/390 which, in version V2R8 at
least, doesn't have snprintf()


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@94895 13f79535-47bb-0310-9956-ffa450edef68