QBenchlib: stop exporting {begin,end}BenchmarkMeasurement

I don't see anything using them in any unit test or other Qt module, and
they are marked internal.

Change-Id: I3c79b7e08fa346988dfefffd172028dae6d9e1ba
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/testlib/qbenchmark_p.h

@@ -154,8 +154,8 @@ namespace QTest
     void setIterationCountHint(int count);
     void setIterationCount(int count);
 
-    Q_TESTLIB_EXPORT void beginBenchmarkMeasurement();
-    Q_TESTLIB_EXPORT quint64 endBenchmarkMeasurement();
+    void beginBenchmarkMeasurement();
+    quint64 endBenchmarkMeasurement();
 }
 
 QT_END_NAMESPACE