Declare pthread_atfork in unistd.h for UNIX98 (bug 20044).

For UNIX98 (only), unistd.h should declare pthread_atfork, but that
declaration is missing.  This patch adds it.

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

	[BZ #20044]
	* posix/unistd.h [__USE_UNIX98 && !__USE_XOPEN2K]
	(pthread_atfork): New prototype.
	* conform/Makefile (test-xfail-UNIX98/unistd.h/conform): Remove
	variable.