Tracker: move common mavlink camera handling up

AntennaTracker/GCS_Mavlink.cpp

@@ -894,3 +894,9 @@ Compass *GCS_MAVLINK_Tracker::get_compass() const
 {
     return &tracker.compass;
 }
+
+/* dummy methods to avoid having to link against AP_Camera */
+void AP_Camera::control_msg(mavlink_message_t const*) {}
+void AP_Camera::configure(float, float, float, float, float, float, float) {}
+void AP_Camera::control(float, float, float, float, float, float) {}
+/* end dummy methods to avoid having to link against AP_Camera */

AntennaTracker/GCS_Mavlink.h

@@ -19,6 +19,7 @@ protected:
     Compass *get_compass() const override;
     AP_Mission *get_mission() override { return nullptr; };
     AP_Rally *get_rally() const override { return nullptr; };
+    AP_Camera *get_camera() const override { return nullptr; };
     AP_ServoRelayEvents *get_servorelayevents() const override { return nullptr; }
     AP_GPS *get_gps() const override;