Tools: add dummy AP::camera method to avoid linking camera

Tools/Replay/Replay.cpp

@@ -950,6 +950,7 @@ const struct AP_Param::GroupInfo        GCS_MAVLINK::var_info[] = {
 GCS_Dummy _gcs;
 
 // dummy methods to avoid linking with these libraries
+AP_Camera *AP::camera() { return nullptr; }
 void AP_Camera::send_feedback(mavlink_channel_t) {}
 void AP_Camera::control(float, float, float, float, float, float) {}
 void AP_Camera::configure(float, float, float, float, float, float, float) {}