AP_RPM: add override keyword where required

libraries/AP_RPM/RPM_PX4_PWM.h

@@ -29,7 +29,7 @@ public:
     ~AP_RPM_PX4_PWM(void);
     
     // update state
-    void update(void);
+    void update(void) override;
 
 private:
     int _fd = -1;

libraries/AP_RPM/RPM_Pin.h

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