[PATCH] libstdc++: Fix static_assert.

[Jonathan Wakely]

On 16 January 2016 at 22:47, Torvald Riegel wrote:
> This adds a missing string argument to a call to static_assert, thus not
> making it depend on c++1z extensions.  This fixes the build breakage on
> mingw introduced in 232454.
>
> Tested on x86_64-linux.  OK?

OK.