simulator: use usual type of timestamp

src/modules/simulator/gpssim/gpssim.cpp

@@ -270,7 +270,7 @@ GPSSIM::receive(int timeout)
 	simulator::RawGPSData gps;
 	sim->getGPSSample((uint8_t *)&gps, sizeof(gps));
 
-	static int64_t timestamp_last = 0;
+	static uint64_t timestamp_last = 0;
 
 	if (gps.timestamp != timestamp_last) {
 		_report_gps_pos.timestamp = hrt_absolute_time();

src/modules/simulator/simulator.h

@@ -115,7 +115,7 @@ struct RawAirspeedData {
 
 #pragma pack(push, 1)
 struct RawGPSData {
-	int64_t timestamp;
+	uint64_t timestamp;
 	int32_t lat;
 	int32_t lon;
 	int32_t alt;