Define TSVTX in tar.h for older POSIX (bug 16978).

Define TSVTX in tar.h for older POSIX (bug 16978).

As noted in bug 16978, older POSIX versions include

in the specified contents of <tar.h>, with only the 2001 edition
introducing the notion of XSI-conditional definitions and conditioning
that definition.  Thus, this macro should be defined for
!__USE_XOPEN2K as well as for __USE_XOPEN, and this patch duly defines
it in that case.  Tested x86_64.

	[BZ #16978]
	* posix/tar.h [!__USE_XOPEN2K] (TSVTX): Define macro.
	* conform/Makefile (test-xfail-POSIX/tar.h/conform): Remove
	variable.