Fix compiler warning about signed/unsigned conversion.

```
src/feature/stats/rephist.c: In function ‘overload_happened_recently’:
src/feature/stats/rephist.c:215:21: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
   if (overload_time > approx_time() - 3600 * n_hours) {
```
from https://gitlab.torproject.org/tpo/core/tor/-/issues/40341#note_2729364

src/feature/stats/rephist.c

@@ -209,7 +209,7 @@ static overload_stats_t overload_stats;
 
 /** Return true if this overload happened within the last `n_hours`. */
 static bool
-overload_happened_recently(time_t overload_time, unsigned n_hours)
+overload_happened_recently(time_t overload_time, int n_hours)
 {
   /* An overload is relevant if it happened in the last 72 hours */
   if (overload_time > approx_time() - 3600 * n_hours) {