Makefile: add extra cls DENCODER_DEPS in the cls makefile, not rgw

They belong next to the class.  Also, if rgw isn't built, ceph-dencoder
will fail to link.

Fixes: #6352
Tested-by: Haomai Wang <haomaiwang@gmail.com>
Signed-off-by: Sage Weil <sage@inktank.com>