POSIX: change pthreads to px4_tasks in simulator

The simulator was using pthread APIs directly so calls to px4_getpid()
would fail since the task ID was not known. Changed simulator to use
px4_task_spawn_cmd.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/modules/simulator/simulator.h

@@ -276,7 +276,7 @@ private:
 	void send_mavlink_message(const uint8_t msgid, const void *msg, uint8_t component_ID);
 	void update_sensors(struct sensor_combined_s *sensor, mavlink_hil_sensor_t *imu);
 	void update_gps(mavlink_hil_gps_t *gps_sim);
-	static void *sending_trampoline(void *);
+	static int sending_trampoline(int argc, char *argv[]);
 	void send();
 #endif
 };