tar: restore macros that are used in some cases

* src/tar.c (LOW_DENSITY_NUM, MID_DENSITY_NUM, HIGH_DENSITY_NUM):
Restore these macros, undoing the previous change to this file.
The macros are used after all, in some cases.  Sorry about that.

src/tar.c

@@ -1142,6 +1142,20 @@ add_file_id (const char *filename)
   file_id_list = p;
 }
 
+/* Default density numbers for [0-9][lmh] device specifications */
+
+#ifndef LOW_DENSITY_NUM
+# define LOW_DENSITY_NUM 0
+#endif
+
+#ifndef MID_DENSITY_NUM
+# define MID_DENSITY_NUM 8
+#endif
+
+#ifndef HIGH_DENSITY_NUM
+# define HIGH_DENSITY_NUM 16
+#endif
+
 static void
 update_argv (const char *filename, struct argp_state *state)
 {