Fix compiler warning about implict cast to double

Change-Id: I80497efedebed9579882d31d3eda13bd88c80c94
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@qt.io>

tests/auto/corelib/global/qrandomgenerator/tst_qrandomgenerator.cpp

@@ -885,7 +885,7 @@ void tst_QRandomGenerator::stdUniformRealDistribution_data()
     newRow(0, 1);   // canonical
     newRow(0, 200);
     newRow(0, numeric_limits<quint32>::max() + 1.);
-    newRow(0, numeric_limits<quint64>::max() + 1.);
+    newRow(0, double(numeric_limits<quint64>::max()) + 1.);
     newRow(-1, 1.6);
 }