rdiff-backup: set meta.platforms

pkgs/tools/backup/rdiff-backup/default.nix

@@ -21,6 +21,7 @@ stdenv.mkDerivation {
     description = "backup system trying to combine best a mirror and an incremental backup system";
     homepage = http://rdiff-backup.nongnu.org/;
     license = stdenv.lib.licenses.gpl2;
+    platforms = stdenv.lib.platforms.all;
     maintainers = with stdenv.lib.maintainers; [ the-kenny ];
   };
 }