Makefile.am: disable building ceph_test_cors when radosgw is not enabled

This test depends on radosgw. Trying to build it without radosgw will
result in a compile error.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Merge pull request #182 from ceph/wip-no-cors-without-rgw

Makefile.am: disable building ceph_test_cors when radosgw is not enabled