build: eliminate GCC8 warnings

modules/imgproc/src/floodfill.cpp

@@ -642,8 +642,15 @@ cvFloodFill( CvArr* arr, CvPoint seed_point,
              CvScalar newVal, CvScalar lo_diff, CvScalar up_diff,
              CvConnectedComp* comp, int flags, CvArr* maskarr )
 {
+#if defined __GNUC__ && __GNUC__ >= 8
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wclass-memaccess"
+#endif
     if( comp )
         memset( comp, 0, sizeof(*comp) );
+#if defined __GNUC__ && __GNUC__ >= 8
+#pragma GCC diagnostic pop
+#endif
 
     cv::Mat img = cv::cvarrToMat(arr), mask = cv::cvarrToMat(maskarr);
     int area = cv::floodFill(img, mask, seed_point, newVal,