another followup to r1828485 needs mod_http2.h

cmake:
   add modules/http2 to includes

legacy:
    add mod_http2.h to install now that mod_proxy.h includes it



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1832855 13f79535-47bb-0310-9956-ffa450edef68