Fix wait3 namespace (bug 21625).

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.

	[BZ #21625]
	* posix/sys/wait.h (strust rusage forward declaration): Change
	[__USE_XOPEN_EXTENDED] conditional to [__USE_XOPEN_EXTENDED &&
	!__USE_XOPEN2K].
	(wait3): Likewise.