rpm,deb: let immutable-object-cache depend on librados2

rpm,deb: let immutable-object-cache depend on librados2

ceph-common depends on librados2, but ceph-immutable-object-cache only
uses librados2's C++ API, so librados2 would suffice.

rpmbuild is able to figure out the .so dependencies, and let
ceph-immutable-object-cache depend on the package which offers
librados2, but we'd better make sure immutable-object-cache is linked
against the librados2 of the same version, to ensure that librados2
offers the correct C++ symbols. as we only version the C++ API in the
librados2 after nautilus.

Signed-off-by: Kefu Chai <kchai@redhat.com>