Support for OpenSSL 1.1.0:

- The callback function passed to
  SSL_CTX_sess_set_get_cb() now needs the
  session id argument to be const.
  So constify the session id.

Backport of r1735883 from trunk.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x-openssl-1.1.0-compat@1755850 13f79535-47bb-0310-9956-ffa450edef68