QBenchmarkResult is larger than a void*, so holding them in a QList is
needlessly inefficient. Worse, the code could come to depend on the
fragile property of (inefficient) QLists that references to elements
therein never are invalidated.

Also saves ~1.2KiB of text size on GCC 4.9 optimized C++11 AMD64
Linux builds.

Change-Id: I0c99e591bb9b4405aa1bb78ec095dcaf9277993f
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>