Fix open_memstream namespace (bug 18498)

[Roland McGrath]

> +/* The prototype needs repeating instead of using __typeof to use
> +   __THROW in C++.  */
> +extern FILE *__open_memstream (char **, size_t *) __THROW __wur;
> +libc_hidden_proto (__open_memstream)

Does "extern __typeof (open_memstream) __open_memstream __THROW __wur;" not
work?  That would be less duplication anyway.  Regardless, the comment
reads as odd in its mention of C++ since there is C++ code in libc to be
calling this.

Substantively the change looks fine to me.