proper signature export (amending b50f8cc8c08e);

--- a/src/Pure/System/isabelle_system.ML	Sun May 23 22:46:30 2021 +0200
+++ b/src/Pure/System/isabelle_system.ML	Sun May 23 23:00:10 2021 +0200
@@ -23,6 +23,8 @@
   val with_tmp_dir: string -> (Path.T -> 'a) -> 'a
   val download: string -> string
   val download_file: string -> Path.T -> unit
+  val decode_base64: string -> string
+  val encode_base64: string -> string
   val isabelle_id: unit -> string
   val isabelle_identifier: unit -> string option
   val isabelle_heading: unit -> string