Rover: fixed constructor ordering build error with gcc-12.2

the constructor order comes from the order in the class

Rover/Parameters.h

@@ -316,13 +316,13 @@ public:
     AP_Beacon beacon;
 #endif
 
-    // Motor library
-    AP_MotorsUGV motors;
-
     // wheel encoders
     AP_WheelEncoder wheel_encoder;
     AP_WheelRateControl wheel_rate_control;
 
+    // Motor library
+    AP_MotorsUGV motors;
+
     // steering and throttle controller
     AR_AttitudeControl attitude_control;