Missing including of nonfree module to support the use of SURF

detector/descriptor.

doc/tutorials/features2d/feature_detection/feature_detection.rst

@@ -31,6 +31,7 @@ This tutorial code's is shown lines below. You can also download it from `here <
    #include "opencv2/core/core.hpp"
    #include "opencv2/features2d/features2d.hpp"
    #include "opencv2/highgui/highgui.hpp"
+   #include "opencv2/nonfree/nonfree.hpp"
 
    using namespace cv;
 

doc/tutorials/features2d/feature_flann_matcher/feature_flann_matcher.rst

@@ -28,6 +28,7 @@ This tutorial code's is shown lines below. You can also download it from `here <
    #include "opencv2/core/core.hpp"
    #include "opencv2/features2d/features2d.hpp"
    #include "opencv2/highgui/highgui.hpp"
+   #include "opencv2/nonfree/nonfree.hpp"
 
    using namespace cv;
 

doc/tutorials/features2d/feature_homography/feature_homography.rst

@@ -30,6 +30,7 @@ This tutorial code's is shown lines below. You can also download it from `here <
    #include "opencv2/features2d/features2d.hpp"
    #include "opencv2/highgui/highgui.hpp"
    #include "opencv2/calib3d/calib3d.hpp"
+   #include "opencv2/nonfree/nonfree.hpp"
 
    using namespace cv;