Get rid of some clazy warnings in graphs2d/piechart folder

Fix "c++11 range-loop might detach Qt container" warnings.

Pick-to: 6.9
Change-Id: I3a61992a654fb09838ae005a67c0085eb8c1421c
Reviewed-by: Jere Tuliniemi <jere.tuliniemi@qt.io>
(cherry picked from commit 59d8120076)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/graphs2d/piechart/qpieseries.cpp

@@ -555,7 +555,8 @@ QPieSlice *QPieSeries::at(qsizetype index)
 */
 QPieSlice *QPieSeries::find(const QString &label)
 {
-    for (QPieSlice *slice : slices()) {
+    auto slicelist = slices();
+    for (QPieSlice *slice : std::as_const(slicelist)) {
         if (slice->label() == label)
             return slice;
     }