Fixes ion_timestamp_to_time_t on Windows (#170)
Fixes ion_timestamp_to_time_t on Windows (#170)

The conversion originally used `_mktime64` which expects a `struct tm`
that is in local time, but for `ION_TIMESTAMP` the representation is
always UTC, so we should've been using `_mkgmtime64`.
amznlabs/ion-c,amzn/ion-c,amzn/ion-c,amzn/ion-c,amznlabs/ion-c,amzn/ion-c,amznlabs/ion-c,amznlabs/ion-c