chore(pivot): Pass proper parameters in flatten function

Author: dkamburov

projects/igniteui-angular/src/lib/grids/pivot-grid/pivot-util.ts

@@ -173,7 +173,7 @@ export class PivotUtil {
             acc.push(item);
           if (item.childLevel) {
             item.expandable = true;
-            acc = acc.concat(this.flatten(item.childLevel, lvl + 1));
+            acc = acc.concat(this.flatten([item.childLevel], lvl + 1));
           }
           return acc;
         }, []);