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/msg.h b/libc/include/sys/msg.h
index 1a6d30d..73965f2 100644
--- a/libc/include/sys/msg.h
+++ b/libc/include/sys/msg.h
@@ -29,6 +29,7 @@
 #ifndef _SYS_MSG_H_
 #define _SYS_MSG_H_
 
+#include <sys/cdefs.h>
 #include <linux/msg.h>
 
 #endif /* _SYS_MSG_H_ */