Add license matching upstream (https://github.com/shadow-maint/shadow)

subid_maximal.c

+*
+* SPDX-FileCopyrightText: 2020 Anders Blomdell, <anders.blomdell@gmail.com> 
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
 #include <stdlib.h>
 #include <pwd.h>
 #include <shadow/subid.h>