continue fixing nonfree elements

fixed dependency of stitching module on xfeatures2d module as optional
fixed the initModule_xfeatures2d function that was called in module stitching since it is in another namespace than the standard cv one.

modules/world/src/world_init.cpp

@@ -52,7 +52,7 @@ bool cv::initAll()
     && initModule_features2d()
 #endif
 #ifdef HAVE_OPENCV_XFEATURES2D
-    && initModule_xfeatures2d()
+    && xfeatures2d::initModule_xfeatures2d()
 #endif
     ;
 }