Fixing missing header compilation problems

samples/c/inpaint.cpp

 #include "opencv2/highgui/highgui.hpp"
 #include "opencv2/imgproc/imgproc_c.h"
 
+#include <stdio.h>
+
 IplImage* inpaint_mask = 0;
 IplImage* img0 = 0, *img = 0, *inpainted = 0;
 CvPoint prev_pt = {-1,-1};

samples/c/letter_recog.cpp

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