ceph.spec.in: do not build with system pmdk by default

ceph.spec.in: do not build with system pmdk by default

we need to use libpmem 1.10 in #40493.

without enabling the module stream offering libpmem 1.9.2, we can only
have access to libpmem 1.6.1. and fedora 33 only has libpmem 1.9
packaged. the same applies to openSUSE Tumbleweed and openSUSE Leap. so
let's stop using libpmem packaged by distro by default, until these
distros include libpmem 1.10.

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