Fix sigwait namespace (bug 21550)

[Zack Weinberg]

On Mon, Jun 5, 2017 at 11:57 AM, Joseph Myers <joseph@codesourcery.com> wrote:
> sigwait was added to POSIX in 1995; it is not in older POSIX versions
> or XPG4 / XPG4.2.  Thus it should be declared only if
> __USE_POSIX199506; signal.h wrongly declares it for __USE_POSIX.
> (sigwaitinfo is correctly conditioned on __USE_POSIX199309.)  This
> patch fixes the condition on the declaration of sigwait.

This is OK.

zw