Implement manual control button reserved function for ArduSub plugin.

src/FirmwarePlugin/APM/APMFirmwarePlugin.h

@@ -79,7 +79,7 @@ public:
     bool        setFlightMode(const QString& flightMode, uint8_t* base_mode, uint32_t* custom_mode) final;
     bool        isGuidedMode(const Vehicle* vehicle) const final;
     void        pauseVehicle(Vehicle* vehicle);
-    int         manualControlReservedButtonCount(void) final;
+    int         manualControlReservedButtonCount(void);
     bool        adjustIncomingMavlinkMessage(Vehicle* vehicle, mavlink_message_t* message) final;
     void        adjustOutgoingMavlinkMessage(Vehicle* vehicle, mavlink_message_t* message) final;
     void        initializeVehicle(Vehicle* vehicle) final;

src/FirmwarePlugin/APM/ArduSubFirmwarePlugin.cc

@@ -47,3 +47,8 @@ ArduSubFirmwarePlugin::ArduSubFirmwarePlugin(void)
     supportedFlightModes << APMSubMode(APMSubMode::ALT_HOLD  ,true);
     setSupportedModes(supportedFlightModes);
 }
+
+int ArduSubFirmwarePlugin::manualControlReservedButtonCount(void)
+{
+    return 0;
+}

src/FirmwarePlugin/APM/ArduSubFirmwarePlugin.h

@@ -67,6 +67,7 @@ public:
     ArduSubFirmwarePlugin(void);
 
     // Overrides from FirmwarePlugin
+    int manualControlReservedButtonCount(void);
 
 };