Fix windows build warnings

samples/cpp/pca.cpp

@@ -176,7 +176,7 @@ int main(int argc, char** argv)
     // display until user presses q
     imshow(winName, reconstruction);
 
-    char key = 0;
+    int key = 0;
     while(key != 'q')
         key = waitKey();
 

samples/cpp/simpleflow_demo.cpp

@@ -86,7 +86,7 @@ static void run(int argc, char** argv) {
 
   Mat flow;
 
-  float start = getTickCount();
+  float start = (float)getTickCount();
   calcOpticalFlowSF(frame1, frame2, 
                     flow,
                     3, 2, 4, 4.1, 25.5, 18, 55.0, 25.5, 0.35, 18, 55.0, 25.5, 10);
@@ -156,7 +156,7 @@ static float calc_rmse(Mat flow1, Mat flow2) {
       }
     }
   }
-  return sqrt(sum / (1e-9 + counter));
+  return (float)sqrt(sum / (1e-9 + counter));
 }
 
 static void eval(int argc, char** argv) {