randstruct: Whitelist struct security_hook_heads cast

The LSM initialization routines walk security_hook_heads as an array
of struct list_head instead of via names to avoid a ton of needless
source. Whitelist this to avoid the false positive warning from the
plugin:

security/security.c: In function ‘security_init’:
security/security.c:59:20: note: found mismatched op0 struct pointer types: ‘struct list_head’ and ‘struct security_hook_heads’

  struct list_head *list = (struct list_head *) &security_hook_heads;
                    ^

Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: James Morris <james.l.morris@oracle.com>
Signed-off-by: NKees Cook <keescook@chromium.org>

scripts/gcc-plugins/randomize_layout_plugin.c

@@ -46,6 +46,8 @@ struct whitelist_entry {
 };
 
 static const struct whitelist_entry whitelist[] = {
+	/* walk struct security_hook_heads as an array of struct list_head */
+	{ "security/security.c", "list_head", "security_hook_heads" },
 	{ }
 };