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.