docs: add license for spinlock.c

Author: auctumnus

src/kernel/spinlock.c

@@ -1,3 +1,9 @@
+/*
+ * SPDX-FileCopyrightText: 2026 ukoOS Contributors
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
 #include <hart_locals.h>
 #include <panic.h>
 #include <spinlock.h>