tracegen: fix inefficient loops

Take the loop variable by cref.

Amends 83effb3a3f.

Pick-to: 6.5
Coverity-Id: 404701
Change-Id: I1266dd1feb16b3842a93169e554a6563861f5a14
Reviewed-by: Antti Määttä <antti.maatta@qt.io>

src/tools/tracegen/helpers.h

@@ -24,7 +24,7 @@ template <typename T>
 static QString aggregateListValues(int value, const QList<T> &list)
 {
     QStringList values;
-    for (auto l : list) {
+    for (const T &l : list) {
         if (l.value == value)
             values << l.name;
     }