Add copyright header

Author: sunkup

lib/src/main/java/at/bitfire/cert4android/CertStore.kt

@@ -1,3 +1,13 @@
+/*
+ * Copyright © All Contributors. See LICENSE and AUTHORS in the root directory for details.
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ *
+ * SPDX-License-Identifier: MPL-2.0
+ */
+
 package at.bitfire.cert4android
 
 import kotlinx.coroutines.flow.StateFlow