Fix open_memstream namespace (bug 18498)

[Roland McGrath]

> > The C++ code is in testcases, which produce an "expected initializer 
> > before 'throw'" error if you use the typeof construct (that is, C++ throw 
> > () only works with an actual function declarator, not when typeof is used 
> > to give something a function type).  Hence the comment, to explain why 
> > __typeof isn't used.
> 
> OK.  It would not have looked so odd to me if the comment said "C++ tests".
> Perhaps also some sort of XXX/TODO comment about how this should go away
> when we one day stop building tests with the internal headers.

Or the decl could just use typeof and be under #ifdef _LIBC, no?