Merge r1422855 from trunk:

Install cache_common.h as required by mod_cache.h. Allows external modules
to use mod_cache.
Submitted by Niklas Edmundsson.

Submitted by: minfrin
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1422880 13f79535-47bb-0310-9956-ffa450edef68