Fix stdlib.h rand_r namespace (bug 20074).

Fix stdlib.h rand_r namespace (bug 20074).

stdlib.h declares rand_r if __USE_POSIX; i.e., POSIX.1:1990.  But
rand_r was added along with threads, so the condition should be
__USE_POSIX199506.  This patch corrects the condition.

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

	[BZ #20074]
	* stdlib/stdlib.h (rand_r): Declare if [__USE_POSIX199506], not if
	[__USE_POSIX].
	* conform/Makefile (test-xfail-XPG4/stdlib.h/conform): Remove
	variable.