Revert "AP_Vehicle: Removed unused airspeed_min and airspeed_max as now part of AP_Airspeed."

This reverts commit bf7ab052c1.

libraries/AP_Vehicle/AP_Vehicle.h

@@ -33,6 +33,8 @@ public:
         AP_Int8 throttle_slewrate;
         AP_Int8 throttle_cruise;
         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;