chore: Fix license headers to GPLv3

Inkay's been GPLv3 for a while and it's fine to relicense old ISC code to GPL

src/config.cpp

@@ -1,6 +1,19 @@
-//
-// Created by ash on 10/12/22.
-//
+/*  Copyright 2022 Pretendo Network contributors <pretendo.network>
+    Copyright 2022 Ash Logan <ash@heyquark.com>
+
+    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 3 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, see <http://www.gnu.org/licenses/>.
+*/
 
 #include "config.h"