conformtest: Correct XOPEN2K stdarg.h expectations.

The conform/ test expectations for stdarg.h were wrongly missing an
expectation of va_copy for XOPEN2K (based on C99, so including that
macro).  This patch fixes this.

Tested for x86_64 and x86.

	* conform/data/stdarg.h-data [XOPEN2K] (va_copy): Require macro.
	* conform/Makefile (test-xfail-XOPEN2K/stdarg.h/conform): Remove
	variable.