AP_RPM: add missing override keywords

libraries/AP_RPM/RPM_SITL.h

@@ -27,7 +27,7 @@ public:
     AP_RPM_SITL(AP_RPM &ranger, uint8_t instance, AP_RPM::RPM_State &_state);
 
     // update state
-    void update(void);
+    void update(void) override;
 private:
     SITL::SITL *sitl;    
     uint8_t instance;