crimson/os/seastore/lba_manager/btree/lba_btree: fix FTBFS on gcc 9

crimson/os/seastore/lba_manager/btree/lba_btree: fix FTBFS on gcc 9

gcc-9 doesn't seem to consider interator nothrow move constructible with
the default move constructor implementation yielding the following build
failure:

m/el8/BUILD/ceph-17.0.0-7373-gfc349212/src/seastar/include/seastar/core/future.hh:584:58: error: static assertion failed: Types must be no-throw move constructible
  584 |     static_assert(std::is_nothrow_move_constructible<T>::value,

Signed-off-by: Samuel Just <sjust@redhat.com>