tree-wide: use the usual SPDX header for our own files

src/basic/memory-util.c

@@ -1,3 +1,5 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+
 #include <unistd.h>
 
 #include "memory-util.h"

src/basic/sort-util.c

@@ -1,3 +1,5 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+
 #include "sort-util.h"
 #include "alloc-util.h"
 

src/basic/static-destruct.h

@@ -1,3 +1,5 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+
 #pragma once
 
 #include "alloc-util.h"