QTransform benchmark: measure what was promised

The function name suggests that the *= operator is to be benchmarked,
not simple multiplication.

Use the correct operator.

Pick-to: 6.3 6.2 5.15
Change-Id: I6718e8aea640a153083858b39963199e7bab26e9
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

tests/benchmarks/gui/painting/qtransform/tst_qtransform.cpp

@@ -335,7 +335,7 @@ void tst_QTransform::operatorMultiplyEqualScalar()
     QFETCH(QTransform, transform);
     QTransform x = transform;
     QBENCHMARK {
-        x * 3;
+        x *= 3;
     }
 }