expose TSVTX unconditionally in tar.h

as noted in Austin Group issue #1236, the XSI shading for TSVTX is
misplaced in the html version of the standard; it was only supposed to
be on the description text. the intent was that the definition always
be visible, which is reflected in the pdf version of the standard.

this reverts commits d93c0740 and
729fef0a.

include/tar.h

 #ifndef	_TAR_H
 #define	_TAR_H
 
-#include <features.h>
-
 #define TSUID   04000
 #define TSGID   02000
-#if defined(_GNU_SOURCE) || defined(_BSD_SOURCE) || defined(_XOPEN_SOURCE)
 #define TSVTX   01000
-#endif
 #define TUREAD  00400
 #define TUWRITE 00200
 #define TUEXEC  00100