Silence the following warnings:

  ```
  /mruby/mrbgems/mruby-time/src/time.c:260:55: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    if ((MRB_INT_MAX > MRB_TIME_MAX && i > 0 && i > MRB_TIME_MAX) ||
                                                  ^
  ```