cmake: enable compile-time fmt format string checking

seastar checks log format strings at compile time when
Seastar_LOGGER_COMPILE_TIME_FMT is on; the option is gated on
fmt_VERSION >= 9.0.0. ceph builds fmt via add_subdirectory(), which
leaves the version in fmt's own FMT_VERSION and never sets the lowercase
fmt_VERSION the option reads, so the check was silently off for crimson.
read the version off the fmt target and expose it so the option turns
on.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>