Silence the following warning:

  ```
  /mruby/mrbgems/mruby-time/src/time.c:258:60: warning: result of comparison of constant -9223372036854775808 with expression of type 'mrb_int' (aka 'int') is always false [-Wtautological-constant-out-of-range-compare]
        if ((mrb_time_int)i > MRB_TIME_MAX || MRB_TIME_MIN > i) {
                                              ~~~~~~~~~~~~ ^ ~
  ```