core: add spdx header to all-units.h

The specific header file is probably not copyrightable anyway, since
it's so trivial, but let's still add the SPDX header line so that a
systematic check for the line does't spit out this header needlessly.

src/core/all-units.h

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
 #pragma once
 
 #include "unit.h"