Fix linknamespace getdate_err handling.

Some linknamespace tests were failing because of references to
getdate_err.  References to this symbol should be allowed for XPG4 and
later XSI standards; it was missing from the whitelist of data
symbols.  This patch adds it to that list.

Tested for x86_64.

	* conform/list-header-symbols.pl (%extra_syms): Add getdate_err
	for XPG4, UNIX98, XOPEN2K and XOPEN2K8.
	* conform/Makefile (test-xfail-XOPEN2K/pthread.h/linknamespace):
	Remove variable.
	(test-xfail-XOPEN2K/time.h/linknamespace): Likewise.
	(test-xfail-XOPEN2K8/pthread.h/linknamespace): Likewise.
	(test-xfail-XOPEN2K8/time.h/linknamespace): Likewise.