Fix wait3 namespace (bug 21625)

[Florian Weimer]

On 06/19/2017 06:17 PM, Joseph Myers wrote:
> The wait3 function was removed in the 2001 edition of POSIX.
> sys/wait.h wrongly declares it for the 2001 and 2008 editions of POSIX
> when XSI features are enabled.  This patch fixes the conditionals.
> 
> Tested for x86_64.
> 
> 2017-06-19  Joseph Myers  <joseph@codesourcery.com>
> 
> 	[BZ #21625]
> 	* posix/sys/wait.h (strust rusage forward declaration): Change
> 	[__USE_XOPEN_EXTENDED] conditional to [__USE_XOPEN_EXTENDED &&
> 	!__USE_XOPEN2K].
> 	(wait3): Likewise.

Looks okay to me based on your explanation.

Thanks,
Florian