add reqiured includes to the "Load, Modify, and Save an Image" tutorial

resolves #6944

doc/tutorials/introduction/load_save_image/load_save_image.rst

@@ -28,6 +28,8 @@ Here it is:
 
    #include <cv.h>
    #include <highgui.h>
+   #include <opencv2/imgproc/imgproc.hpp>
+   #include <stdio.h>
 
    using namespace cv;