appveyor: labs(time_t) is not allowed

Mike points out that appveyor is failing with

../src/core/or/channeltls.c:1768:7: error: absolute value function 'labs' given an argument of type 'time_t' {aka 'long long int'} but has parameter of type 'long int' which may cause truncation of value [-Werror=absolute-value]
 1768 |   if (labs(now - chan->conn->handshake_state->sent_versions_at) < 180) {
      |       ^~~~

That code has been there for ages, though. Looks like this is a new compiler warning rather than a new bug. It is a c issue, though, anywhere that time_t is bigger than long.