base64: Restore original license header.

When incorporating a copyleft licensed program with a lax licensed program,
it is correct to preserve the original license notice.  See also:
  https://www.softwarefreedom.org/resources/2007/gpl-non-gpl-collaboration.html

* guix/base64.scm: Restored original license header.

guix/base64.scm

@@ -6,8 +6,6 @@
 ;;
 ;; Some optimizations made by Ludovic Courtès <ludo@gnu.org>, 2015.
 ;;
-;; Copyright © 2009, 2010 Göran Weinholt <goran@weinholt.se>
-;;
 ;; This program is free software: you can redistribute it and/or modify
 ;; it under the terms of the GNU General Public License as published by
 ;; the Free Software Foundation, either version 3 of the License, or
@@ -20,6 +18,30 @@
 ;;
 ;; You should have received a copy of the GNU General Public License
 ;; along with this program.  If not, see <http://www.gnu.org/licenses/>.
+;;
+;; This file incorporates work covered by the following copyright and  
+;; permission notice:
+;;
+;;   Copyright © 2009, 2010 Göran Weinholt <goran@weinholt.se>
+;;
+;;   Permission is hereby granted, free of charge, to any person obtaining a
+;;   copy of this software and associated documentation files (the "Software"),
+;;   to deal in the Software without restriction, including without limitation
+;;   the rights to use, copy, modify, merge, publish, distribute, sublicense,
+;;   and/or sell copies of the Software, and to permit persons to whom the
+;;   Software is furnished to do so, subject to the following conditions:
+;;
+;;   The above copyright notice and this permission notice shall be included in
+;;   all copies or substantial portions of the Software.
+;;
+;;   THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+;;   IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+;;   FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+;;   THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+;;   LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+;;   FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+;;   DEALINGS IN THE SOFTWARE.
+
 #!r6rs
 
 ;; RFC 4648 Base-N Encodings