[PATCH] Fix unused private field warning in stdexcept after r207695

[Joerg Sonnenberger]

On Sun, Jan 04, 2015 at 03:24:35PM +0000, Dimitry Andric wrote:
> However, there isn't any _LIBCPP_UNUSED define in __config yet, so that would have to be added, like in this proposed change.

Shouldn't it actually be attribute *used*?

Joerg