Add SPDX header

Signed-off-by: Andy Scherzinger <info@andy-scherzinger.de>

app/src/main/res/values/strings.xml

@@ -1,4 +1,10 @@
 <?xml version="1.0" encoding="utf-8"?>
+<!--
+  ~ Nextcloud - Android Client
+  ~
+  ~ SPDX-FileCopyrightText: 2016-2024 Nextcloud GmbH and Nextcloud contributors
+  ~ SPDX-License-Identifier: AGPL-3.0-or-later
+-->
 <resources>
     <string name="about_android">%1$s Android app</string>
     <string name="about_version">version %1$s</string>