timeval: make timediff_t also work on 32bit windows

... by using curl_off_t for the typedef if time_t is larger than 4
bytes.

Reported-by: Gisle Vanem
Bug: https://github.com/curl/curl/commit/b9d25f9a6b3ca791385b80a6a3c3fa5ae113e1e0#co
mmitcomment-25205058
Closes #2019