Plane: add transition class as friend

ArduPlane/Plane.h

@@ -136,6 +136,8 @@ public:
     friend class RC_Channels_Plane;
     friend class Tailsitter;
     friend class Tiltrotor;
+    friend class SLT_Transition;
+    friend class Tailsitter_Transition;
 
     friend class Mode;
     friend class ModeCircle;