Fixed compilation warnings in samples:

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

samples/cpp/autofocus.cpp

@@ -28,7 +28,9 @@
 #include <cstdio>
 #include <iostream>
 #include <algorithm>
-#include <opencv2/opencv.hpp>
+#include <opencv2/core.hpp>
+#include <opencv2/imgproc.hpp>
+#include <opencv2/highgui.hpp>
 
 using namespace std;
 using namespace cv;