modules/md/mod_md.h is now a new public API

also used by other modules (currently mod_ssl),
so it must be in the include path. It was
missing for cmake builds.

Note that this change does not yet enable building
mod_md itself using cmake. That part is still
missing.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1828669 13f79535-47bb-0310-9956-ffa450edef68