Satisfy test suite and CLI

xs/xsp/Config.xsp

@@ -41,6 +41,7 @@
         %code{% THIS->apply(*other, true); %};
     std::vector<std::string> get_keys()
         %code{% THIS->keys(&RETVAL); %};
+    std::string get_extrusion_axis();
 };
 
 %name{Slic3r::Config::PrintRegion} class PrintRegionConfig {