QString benchmark: don't use %ls

The vsnprintf we use in QTest doesn't not have a portable %ls:
It accepts wchar_t, so it's UTF-32 on Linux and UTF-16 on Windows

Change-Id: I9ebda1e92b6e8e4dbbb79c6f2e35a833c587a089
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

tests/benchmarks/corelib/text/qstring/tst_bench_qstring.cpp

@@ -299,7 +299,7 @@ void tst_QString::number_double_data()
     };
 
     for (auto &datum : data) {
-        QTest::addRow("%ls, format '%c', precision %d", qUtf16Printable(datum.expected), datum.f,
+        QTest::addRow("%s, format '%c', precision %d", qPrintable(datum.expected), datum.f,
                       datum.p)
                 << datum.d << datum.f << datum.p << datum.expected;
     }