POSIX: px4_getpid() fix

Since the PX4 code uses both px4_task and pthread APIs,
px4_getpid() must be save to call from either context.

On posix, this means we have to always return the pthread ID.

Reverted simulator change of pthread to px4_task

There may have been side effects if this was build for a target that
has process/task scoped file descriptors. It is now safe to call
px4_getpid() from this pthread context with this change for the
posix build for px4_getpid().

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 int sending_trampoline(int argc, char *argv[]);
+	static void *sending_trampoline(void *);
 	void send();
 #endif
 };