apache-httpd: Use authn_core for version >= 2.3.

Beginning with version 2.3, the authn were refactored. As a result, authn_alias
is now part of the new module authn_core, so let's use authn_core instead of
authn_alias.

For details please see: http://httpd.apache.org/docs/2.4/upgrading.html#misc



Signed-off-by: aszlig <aszlig@redmoonstudios.org>