Fix 48631_neg test in _GLIBCXX_VERSION_NAMESPACE mode

[François Dumont]

Here is a small patch to fix a test which fails in 
_GLIBCXX_VERSION_NAMESPACE mode.

IMHO it would be better to avoid putting <type_traits> content in 
versioned namespace, no ?

There is of course more work to do, so for now here is the simpler approach.

Ok to commit ?

François


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