Follow up to r1527295:

ssl_engine_dh.c is still in 2.4.x, and (perhaps unexpectedly) this
cmake list supports both trunk and 2.4.x for the time being.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1527570 13f79535-47bb-0310-9956-ffa450edef68