When building Ruby on Ubuntu 22.04 and GCC 11.4.0, the following warning appeared.
And this change has suppressed warning.

```
compiling ../ruby/time.c
../ruby/time.c: In function ‘time_init_parse’:
../ruby/time.c:2650:21: warning: comparison of integer expressions of different signedness: ‘size_t’ {aka ‘long unsigned int’} and ‘int’ [-Wsign-compare]
 2650 |         if (ndigits < TIME_SCALE_NUMDIGITS) {
      |                     ^
../ruby/time.c:2654:26: warning: comparison of integer expressions of different signedness: ‘size_t’ {aka ‘long unsigned int’} and ‘int’ [-Wsign-compare]
 2654 |         else if (ndigits > TIME_SCALE_NUMDIGITS) {
      |
```