allow mod_auth_digest to configure with --srcdir

(added same include path as modules/mappers/config9.m4)
PR:
Obtained from:
Submitted by:
Reviewed by:


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@92880 13f79535-47bb-0310-9956-ffa450edef68