Always include <sys/cdefs.h>.

I made a mistake caught by code review earlier, so let's try to be
safer by default.

This patch also moves all our "forwarding" headers to the guardless
just-include-the-other-thing style that we usually use. (Where we
have a comment explaining where the header comes from, I've kept
that.)

Change-Id: I37342cf5e2563c6a269b2ba61a697069b1c7913b
diff --git a/libc/include/sys/queue.h b/libc/include/sys/queue.h
index b0e6b38..0890838 100644
--- a/libc/include/sys/queue.h
+++ b/libc/include/sys/queue.h
@@ -32,6 +32,8 @@
 #ifndef	_SYS_QUEUE_H_
 #define	_SYS_QUEUE_H_
 
+#include <sys/cdefs.h>
+
 /*
  * This file defines five types of data structures: singly-linked lists,
  * lists, simple queues, tail queues, and circular queues.