src/: mark final class as final

src/: mark final class as final

to silence warnings from clang-10 like:

```
../src/os/bluestore/ceph_aio.h:120:18: warning: class with destructor marked 'final' cannot be inherited from [-Wfinal-dtor-non-final-class]
  ~aio_queue_t() final {
                 ^
../src/os/bluestore/ceph_aio.h:108:8: note: mark 'aio_queue_t' as 'final' to silence this warning
struct aio_queue_t : public io_queue_t {
       ^
1 warning generated.
```

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