Do not declare grantpt, ptsname, unlockpt in stdlib.h for XPG3 (bug 20094).

stdlib.h declares grantpt, ptsname, unlockpt for __USE_XOPEN.  This
patch corrects the condition to __USE_XOPEN_EXTENDED (these functions
are new in XPG4).

Tested for x86_64 and x86 (testsuite, and that installed shared
libraries are unchanged by the patch).

	[BZ #20094]
	* stdlib/stdlib.h (grantpt): Declare if [__USE_XOPEN_EXTENDED],
	not [__USE_XOPEN].
	(unlockpt): Likewise.
	(ptsname): Likewise.
	* conform/Makefile (test-xfail-XPG3/stdlib.h/conform): Remove
	variable.