licensing: add forgotten spdx headers

Those are all "our" files, but we forgot to add the headers,
most likely because of non-standard file extensions.

src/basic/missing_securebits.h

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
 #include <linux/securebits.h>