samples: use findFile() in "cpp"

samples/cpp/dft.cpp

@@ -14,12 +14,12 @@ static void help()
     printf("\nThis program demonstrated the use of the discrete Fourier transform (dft)\n"
            "The dft of an image is taken and it's power spectrum is displayed.\n"
            "Usage:\n"
-            "./dft [image_name -- default ../data/lena.jpg]\n");
+            "./dft [image_name -- default lena.jpg]\n");
 }
 
 const char* keys =
 {
-    "{help h||}{@image|../data/lena.jpg|input image file}"
+    "{help h||}{@image|lena.jpg|input image file}"
 };
 
 int main(int argc, const char ** argv)
@@ -32,7 +32,7 @@ int main(int argc, const char ** argv)
         return 0;
     }
     string filename = parser.get<string>(0);
-    Mat img = imread(filename, IMREAD_GRAYSCALE);
+    Mat img = imread(samples::findFile(filename), IMREAD_GRAYSCALE);
     if( img.empty() )
     {
         help();