cmake: make the linkage to pmem::pmemobj public

cmake: make the linkage to pmem::pmemobj public

tools/ceph-dencoder/rbd_types.cc includes Types.h which in turn includes
libpmemobj.h via librbd/cache/pwl/Types.h. and ceph-dencoder pulls in the
rbd_type.cc's linked libraries by linking against rbd_types. but before
this change, rbd_types links against pmem::pmemobj as a PRIVATE library.

so, if we want to pull in rbd_types linkage we should always link
rbd_types as a PUBLIC library. as rbd_types include libpmemobj.h in its
header file.

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