[committed] Limit <stacktrace> header synopsis test to normal namespace

[François Dumont]

Committed as trivial.

     libstdc++: Limit <stacktrace> synopsis test to normal namespace

     libstdc++-v3/ChangeLog

             * testsuite/19_diagnostics/stacktrace/synopsis.cc: Add
             { dg-require-normal-namespace "" }.

François
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