QSignalSpy: fix indexed loop (int instead of qsizetype)

While a signal with more than 2Gi arguments is only a theoretical
possibility, still use the correct index variable for the indexed loop
over this QList<int>.

Pick-to: 6.7 6.6 6.5
Change-Id: I2ed33238c2cd9d2d1c39cd29c988a2adfd821897
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/testlib/qsignalspy.h

@@ -193,7 +193,7 @@ private:
         QMutexLocker locker(&m_mutex);
         QList<QVariant> list;
         list.reserve(args.size());
-        for (int i = 0; i < args.size(); ++i) {
+        for (qsizetype i = 0; i < args.size(); ++i) {
             const QMetaType::Type type = static_cast<QMetaType::Type>(args.at(i));
             if (type == QMetaType::QVariant)
                 list << *reinterpret_cast<QVariant *>(a[i + 1]);