Merge r1200699 from trunk:

Fix installation of conf/extra/proxy-html.conf.

Otherwise httpd does no longer start, because
it is the only extra file required in the main
config at the moment.

Submitted by: rjung
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1200973 13f79535-47bb-0310-9956-ffa450edef68