[godot] Make SpineAtlasResource::clear_native_data const.

spine-godot/spine_godot/SpineAtlasResource.h

@@ -77,7 +77,7 @@ public:
 
 	Array get_normal_maps();
 
-	void clear_native_data() {
+	const void clear_native_data() {
 		this->atlas = nullptr;
 		this->texture_loader = nullptr;
 	}