ceph-object-corpus: update submodule

Update the ceph-object-corpus submodule to pick up the change to
mark cls_rbd_snap as forward incompatible since nautilus. This change
allows us to update ceph-dencoder to allocate fresh instances for each
decode operation instead of reusing existing ones. The uncoming
change in ceph-dencoder will allow us to identify the potential
compatibility break early.

Signed-off-by: Kefu Chai <[email protected]>

Author: tchaikov