crimson/os: mark class with "final" dtor "final"

crimson/os: mark class with "final" dtor "final"

to silence warning like

scheduler.cc:145:30: warning: class with destructor marked 'final' cannot be inherited from [-Wfinal-dtor-non-final-class]
  ~ClassedOpQueueScheduler() final {};
                             ^
/home/jenkins-build/build/workspace/ceph-perf-crimson/ceph-pr/src/crimson/osd/scheduler/scheduler.cc:52:7: note: mark 'ClassedOpQueueScheduler<T>' as 'final' to silence this warning
class ClassedOpQueueScheduler : public Scheduler {
      ^

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