Fixed compilation warnings in samples:

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

samples/cpp/tutorial_code/viz/histo3D.cpp

@@ -1,4 +1,6 @@
-#include <opencv2/opencv.hpp>
+#include <opencv2/core.hpp>
+#include <opencv2/imgproc.hpp>
+#include <opencv2/highgui.hpp>
 #include <iostream>
 
 using namespace std;
@@ -6,6 +8,8 @@ using namespace cv;
 
 #ifdef HAVE_OPENCV_VIZ
 
+#include <opencv2/viz.hpp>
+
 const String keys =
 "{Aide h usage ? help  |     | print this message   }"
 "{@arg1                |     | Full path to color imag (3 channels)}"