docs: restore missing license information

modules/intermediate_format/preprocessing.xqm

 xquery version "3.1";
+
+(:
+ : Copyright (c) 2018 Uwe Sikora
+ : Copyright (c) 2018–2019 Michelle Weidling
+ : Copyright (c) 2020–2021 Stefan Hynek
+ : Copyright (c) 2021 Simon Sendler
+ :
+ : This file is part of intermediate-format.
+ :
+ : intermediate-format 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
+ : (at your option) any later version.
+ :
+ : intermediate-format is distributed in the hope that it will be useful,
+ : but WITHOUT ANY WARRANTY; without even the implied warranty of
+ : MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ : GNU General Public License for more details.
+ :
+ : You should have received a copy of the GNU General Public License
+ : along with intermediate-format.  If not, see <https://www.gnu.org/licenses/>.
+ :)
+
 (:~
  : PREPROCESSING Module ("pre", "http://bdn.edition.de/intermediate_format/preprocessing")
  : *******************************************************************************************