SITL: add override keyword where required

libraries/SITL/SIM_Submarine.h

@@ -34,7 +34,7 @@ public:
     Submarine(const char *home_str, const char *frame_str);
 
     /* update model by one time step */
-    void update(const struct sitl_input &input);
+    void update(const struct sitl_input &input) override;
 
     /* static object creator */
     static Aircraft *create(const char *home_str, const char *frame_str) {