24902 fix breaks on clang

https://travis-ci.org/torproject/tor/jobs/335395212#L1716

src/or/dos.c:277:40: error: implicit conversion loses integer precision: 'long'
      to 'uint32_t' (aka 'unsigned int') [-Werror,-Wshorten-64-to-32]
  num_token = elapsed_time_last_refill * circuit_rate;
            ~ ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~