Overflow might happen easier on Windows

[bvssvni]

See tomaka/clock_ticks#4

clock_ticks is based on this library, so I suspect the same issue applies here.

https://github.com/rust-lang/time/blob/a9b589595eaffd9f5b6a4b53bc89a32246204ac2/src/lib.rs#L251

return (ticks as u64 * 1000000000) / (imp::frequency() as u64);

[huonw]

cc rust-lang/rust#17845, rust-lang/rust#22788

[alexcrichton]

Applied the same fix as in the main repo, thanks!