tst_QAlgorithms: fix compilation with C++11 enabled

GCC refuses to use a merely static const uint array in a constexpr function.

Fix by making the array constexpr if supported by the compiler.

Change-Id: Idd59d3f74f8f4e98aad82bc892f4a6469932df9f
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

tests/auto/corelib/tools/qalgorithms/tst_qalgorithms.cpp

@@ -1027,7 +1027,7 @@ void tst_QAlgorithms::binaryFindOnLargeContainer() const
 }
 
 // alternative implementation of qPopulationCount for comparison:
-static const uint bitsSetInNibble[] = {
+static Q_DECL_CONSTEXPR const uint bitsSetInNibble[] = {
     0, 1, 1, 2, 1, 2, 2, 3,
     1, 2, 2, 3, 2, 3, 3, 4,
 };