fix default feature profile in tar.h

commit d93c0740d86aaf7043e79b942a6c0b3f576af4c8 added use of feature
test macros without including features.h, causing a definition that
should be exposed in the default profile, TSVTX, to appear only when
_XOPEN_SOURCE or higher is explicitly defined.

include/tar.h

@@ -1,6 +1,8 @@
 #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)