Re-license the library as LGPLv2-or-later

This facilitates the use from other projects such as ASL licensed
software.  We think there is no point in others having to re-implement
the GTP specific message parsing/encoding on top of libmnl, which is
LGPL anyway.

Please note that the tools are still licensed under GPL, just the
library has been changed.
diff --git a/src/genl.c b/src/genl.c
index a2826b5..311480c 100644
--- a/src/genl.c
+++ b/src/genl.c
@@ -6,16 +6,16 @@
  * All Rights Reserved
  *
  * 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.
+ * it under the terms of the GNU Lesser General Public License as
+ * published by the Free Software Foundation; either version 2.1 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 Affero General Public License for more details.
+ * GNU Lesser General Public License for more details.
  *
- * You should have received a copy of the GNU General Public License
+ * You should have received a copy of the GNU Lesser General Public License
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  *
  */