fixed compile errors from gcc

samples/cpp/letter_recog.cpp

 #include "opencv2/core/core_c.h"
 #include "opencv2/ml/ml.hpp"
 
+#include <cstdio>
 /*
 The sample demonstrates how to train Random Trees classifier
 (or Boosting classifier, or MLP - see main()) using the provided dataset.

samples/cpp/watershed.cpp

 #include "opencv2/imgproc/imgproc.hpp"
 #include "opencv2/highgui/highgui.hpp"
 
+#include <cstdio>
 #include <iostream>
 
 using namespace cv;