fix minor namespace issue in tar.h

TSVTX is XSI-shaded.

include/tar.h

@@ -3,7 +3,9 @@
 
 #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