seastar: pick up change in fmt submodule

seastar: pick up change in fmt submodule

see https://github.com/ceph/fmt/pull/1

so fmt won't fallback to C++11 if `CMAKE_CXX_STANDARD` is not set

Signed-off-by: Kefu Chai <kchai@redhat.com>
this change is not cherry-picked from master, because

- the seastar submodule in nautilus is not in sync with that in
  master
- master is not using fmt submodule included by seastar anymore
- master is using cmake 3.10, so there is no need to address
  this issue in master on fmt front.