LICENSE: http_ext: fix GPL license version

This is a followup of the previous commit: GH user @songliumeng initially
reported an issue with the GPL license version for event_hdl source file
which was fixed by the previous commit. It turns out the same mistake was
made in http_ext source file: due to a mixup between LGPL and GPL, GPL
version '2.1' was referenced instead of '2'.

Again, clarify that this is indeed GPL by making use of the banner
provided in doc/gpl.txt

This should be backported in 2.8 with b2bb925 ("MINOR: proxy/http_ext:
introduce proxy forwarded option")

src/http_ext.c

@@ -3,11 +3,19 @@
  *
  * Copyright 2022 HAProxy Technologies
  *
- * 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
- * 2.1 of the License, or (at your option) any later version.
+ * 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, Fifth Floor, Boston, MA  02110-1301  USA
  */
 
 #include <haproxy/sample.h>