[PATCH] linux: Fix a non-constant expression in _Static_assert

[Florian Weimer]

* Fangrui Song via Libc-alpha:

> The description needs some adjustment
> https://www.iso-9899.info/n1570.html#6.6p10
>
> 10   An implementation may accept other forms of constant expressions.

Yeah, but the change itself is clearly correct.  Thanks.

Florian