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/stitching/src/matchers.cpp

@@ -49,7 +49,7 @@ using namespace cv::cuda;
 #ifdef HAVE_OPENCV_XFEATURES2D
 #include "opencv2/xfeatures2d.hpp"
 
-static bool makeUseOfNonfree = initModule_xfeatures2d();
+static bool makeUseOfXfeatures2d = xfeatures2d::initModule_xfeatures2d();
 #endif
 
 namespace {