Add a fucntion to expose the internal `struct tm` that is populated
with the GPS date.
This allows for easy syncing the RTC with the GPS time without having
to convert back and forth between `struct tm` and `time_t`.
struct tm now;
minmea_getdate(&now, data, time);
rtc_set_time(&now)
upstream PR: https://github.com/kosma/minmea/pull/45
