Windows 32-bit build broken with introduction of `overload_happened_recently()` in rephist.c

Commit 0a5ecb33 introduced a new function named overload_happened_recently() which does a comparison between signed and unsigned data on 32-bit Windows.

The error is:

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) {

See: https://travis-ci.org/github/ahf/tor-win32/jobs/763284597#L8775-L8777