Mount_Backend: add set_mode virtual method

libraries/AP_Mount/AP_Mount_Backend.h

@@ -46,6 +46,9 @@ public:
     // has_pan_control - returns true if this mount can control it's pan (required for multicopters)
     virtual bool has_pan_control() const = 0;
 
+    // set_mode - sets mount's mode
+    virtual void set_mode(enum MAV_MOUNT_MODE mode) = 0;
+
     // set_roi_target - sets target location that mount should attempt to point towards
     virtual void set_roi_target(const struct Location &target_loc) = 0;