ceph.spec.in: build with header only fmt on RHEL

ceph.spec.in: build with header only fmt on RHEL

because fmt is packaged in EPEL, while librados is packaged
in RHEL, so we cannot have fmt as a runtime dependency of librados.
to address this issue, we should compile librados either with static library
or with header-only library of fmt. but because the fedora packaging
guideline does no encourage us to package static libraries, and it would
be complicated to package both static and dynamic library for fmt.

the simpler solution would be to compile Ceph with the header-only
version of fmt.

in this change, we compile ceph with the header-only version of fmt
on RHEL to address the runtime dependency issue.

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