Fix fractional part of gwTime in case of uplink received via LoRa Basics Station

[beitler]

Whenever an uplink is received via a LoRa Basics Station gateway which doesn't gave GPS time available, the fractional second of the receive time is missing in the reported gwTime of the rx_info element.
Example:

It seems the fractional part of the rxtime (as float) is not properly scaled to nanoseconds before it is cast to int64.

chirpstack-gateway-bridge/internal/backend/basicstation/structs/radio_meta_data.go

Lines 80 to 86 in 6897fe5

	if rxTime := rmd.UpInfo.RxTime; rxTime != 0 {
	sec, nsec := math.Modf(rmd.UpInfo.RxTime)
	if sec != 0 {
	val := time.Unix(int64(sec), int64(nsec))
	pb.RxInfo.GwTime = timestamppb.New(val)
	}
	}

This PR fixes that.

Note: rxtime in Basics Station is the radio receive time (xtime) converted into UTC. This conversion is susceptible to UTC synchronization issues (e.g. NTP not functioning correctly, or large/asymmetric network latencies in case of LNS-based timesync) and therefore must be used with care by any consumer.

[brocaar]

Thanks @beitler 👍