Fixed compilation warnings in samples:

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

samples/cpp/tutorial_code/snippets/core_various.cpp

@@ -1,4 +1,7 @@
-#include <opencv2/opencv.hpp>
+#include <opencv2/core.hpp>
+#include <opencv2/imgproc.hpp>
+#include <opencv2/highgui.hpp>
+#include <opencv2/features2d.hpp>
 
 using namespace std;
 using namespace cv;

samples/cpp/tutorial_code/snippets/imgcodecs_imwrite.cpp

@@ -1,4 +1,4 @@
-#include <opencv2/opencv.hpp>
+#include <opencv2/imgcodecs.hpp>
 
 using namespace cv;
 using namespace std;