HAL_SITL: fixed a warning

libraries/AP_HAL_AVR_SITL/sitl_gps.cpp

@@ -530,8 +530,8 @@ void SITL_State::_update_gps(double latitude, double longitude, float altitude,
 	char c;
     Vector3f glitch_offsets = _sitl->gps_glitch;
 
-	// 5Hz, to match the real config in APM
-	if (hal.scheduler->millis() - gps_state.last_update < 1000/_sitl->gps_hertz) {
+	// run at configured GPS rate (default 5Hz)
+	if ((hal.scheduler->millis() - gps_state.last_update) < (uint32_t)(1000/_sitl->gps_hertz)) {
 		return;
 	}