Fix sntp_get_real_time prototype in header

[earlephilhower]

sntp_get_real_time takes a time_t in the core, but in the header it
was listed as long. Make them both match by changing the header.

[earlephilhower]

This was spotted while doing the newlib 3.3 port where time_t has gone to 64-bit (i.e. a long long) to avoid the year 2038 problem. On the current core, this is just a cleanup and shouldn't change any code.