so, on debian derivatives, we can use the libfmt-dev package for
building Ceph. this change is created in hope to reduce the compile
time.
>= 6.1.2 is specified, as it is the version packaged by ubuntu focal,
which is used for running "make check" and intergration tests.
find_package(fmt 6.0.0 QUIET)
Signed-off-by: Kefu Chai <kchai@redhat.com>