ceph.spec.in: obsolete ceph-libs only on the affected distro

The ceph-libs package existed only on Redhat based distro,
there was e.g. never such a package on SUSE. Therefore: make
sure the 'Obsoletes' is only set on these affected distros.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>