libdpkg: Revert license change to GPL2+

The file lib/dpkg/showpkg.c used to be GPL-2+ with all commits with
copyright by Wichert Akkerman, until commit 6e1eb71ebffbbaca2e4bc when
it got changed to GPL-2 only.

Changes afterwards by:

  Wichert Akkerman (trivial change):

    commit d5e656d9: Removal of an include.

  Adam Heath (trivial changes):

    commit dbcb7449: Reorder an include;
    commit a1d9dc46: Add a missing “void”;
                     Macroify the unused dumpchain to avoid a warning,
                     code entirely removed in commit 07162068.

  Frank Lichtenheld (several changes)

  Pierre Habouzit (trivial change):

    commit 8ad47fe5: Add a 'z' to a format string.

  Guillem Jover (several changes)

So with approval from Frank and myself, let's revert the license change
in 6e1eb71ebffbbaca2e4bc.

Signed-off-by: Frank Lichtenheld <djpig@debian.org>
Signed-off-by: Guillem Jover <guillem@debian.org>

lib/dpkg/showpkg.c

@@ -5,8 +5,9 @@
  * Copyright © 2001 Wichert Akkerman <wakkerma@debian.org>
  *
  * This is free software; you can redistribute it and/or modify
- * it under the terms of version 2 of the GNU General Public
- * License version 2 as published by the Free Software Foundation.
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
  *
  * This is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of