debian: make radosgw require matching version of librados2

...indirectly via ceph-common.  We get bad behavior when they diverge, I
think because of libcommon.la being linked both statically and dynamically.

Fixes: #4997
Backport: cuttlefish, bobtail
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Gary Lowell <gary.lowell@inktank.com>