build: globally set FMT_USE_TZSET=0 for Windows

build: globally set FMT_USE_TZSET=0 for Windows

We're currently setting FMT_USE_TZSET=0 when building libfmt
in order to avoid the _tzset function, which is unavailable
under Mingw:
https://github.com/ceph/ceph/commit/aa5769ecf1d80fc9824280d2e90fd4c61a0e7769

The issue is that it still gets used by fmt/chrono.h, which is
why we'll move this definition to the top level cmake file.

Note that the Windows build is currently failing as a result of
a recent change: https://github.com/ceph/ceph/pull/52590/files

  In file included from ceph/src/common/ceph_time.h:22,
                   from ceph/src/include/encoding.h:31,
                   from ceph/src/include/uuid.h:9,
                   from ceph/src/include/types.h:21,
                   from ceph/src/crush/CrushWrapper.h:14,
                   from ceph/src/crush/CrushCompiler.h:7,
                   from ceph/src/crush/CrushCompiler.cc:4:
  ceph/src/fmt/include/fmt/chrono.h: In lambda function:
  ceph/src/fmt/include/fmt/chrono.h:953:5: error: ‘_tzset’ was
  not declared in this scope; did you mean ‘tzset’?
    953 |     _tzset();
        |     ^~~~~~
        |     tzset

Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>