Use SPDX identifiers in more files

samples/xmpprint.cpp

@@ -1,30 +1,5 @@
-// ***************************************************************** -*- C++ -*-
-/*
- * Copyright (C) 2004-2021 Exiv2 authors
- * This program is part of the Exiv2 distribution.
- *
- * 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 2
- * of the License, or (at your option) any later version.
- *
- * This program 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 this program; if not, write to the Free Software
- * Foundation, Inc., 51 Franklin Street, 5th Floor, Boston, MA 02110-1301 USA.
- */
-
-// xmpprint.cpp
-// Read an XMP from a video or graphic file, parse it and print 
-// all (known) properties.
-// ========================================================================
-// Linux standalone compilation : 
-//      g++ -o xmpprint xmpprint.cpp `pkg-config --cflags --libs exiv2`
-// ========================================================================
+// SPDX-License-Identifier: GPL-2.0-or-later
+// Read an XMP from a video or graphic file, parse it and print  all (known) properties.
 
 #include <exiv2/exiv2.hpp>
 #include <string>