AP_Vehicle: moved roll_limit_cd into AP_Vehicle

will be used by APM_Control

libraries/AP_Vehicle/AP_Vehicle.h

@@ -35,6 +35,7 @@ public:
         AP_Int8 takeoff_throttle_max;
         AP_Int16 airspeed_min;
         AP_Int16 airspeed_max;
+        AP_Int16 roll_limit_cd;
         AP_Int16 pitch_limit_max_cd;
         AP_Int16 pitch_limit_min_cd;        
         AP_Int8  autotune_level;