strcmp(1): add copyright header

I could trace strcmp(1) as far back as
<293436c5ad/src/streq.c>
in my repo.

src/strcmp.c

@@ -1,3 +1,21 @@
+/*
+ * Copyright (c) 2022–2024 DTB <trinity@trinity.moe>
+ * 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 https://www.gnu.org/licenses/.
+ */
+
 #include <stdio.h> /* fprintf(3), stderr */
 #include <stdlib.h> /* EXIT_FAILURE */
 #include <sysexits.h>