add missing license header

.drone.yml

@@ -193,6 +193,6 @@ steps:
 
 ---
 kind: signature
-hmac: 4789cebf9156b2c3cb0f097311ea7620e709e4332f130dcae51a938515dc952e
+hmac: f7ef1e0d3d4fe0a55d43ba0ab5ed6cb5f5c8bf00791464ce7b251a3cdbfd954a
 
 ...

internal/oauth/util.go

@@ -1,3 +1,20 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero 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 Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
 package oauth
 
 import (