Fixed compilation warnings in samples:

- use per-module includes instead of "opencv.hpp"
- unused function warnings
- undefined macro warning

samples/cpp/tutorial_code/xfeatures2D/LATCH_match.cpp

@@ -4,10 +4,12 @@
 
 #ifdef HAVE_OPENCV_XFEATURES2D
 
+#include <opencv2/core.hpp>
+#include <opencv2/imgproc.hpp>
+#include <opencv2/highgui.hpp>
 #include <opencv2/features2d.hpp>
 #include <opencv2/xfeatures2d.hpp>
 #include <opencv2/imgcodecs.hpp>
-#include <opencv2/opencv.hpp>
 #include <vector>
 
 // If you find this code useful, please add a reference to the following paper in your work: