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/timerfd.h b/libc/include/sys/timerfd.h
index 8311f08..0090233 100644
--- a/libc/include/sys/timerfd.h
+++ b/libc/include/sys/timerfd.h
@@ -31,6 +31,7 @@
 
 #include <fcntl.h> /* For O_CLOEXEC and O_NONBLOCK. */
 #include <time.h>
+#include <sys/cdefs.h>
 #include <sys/types.h>
 
 __BEGIN_DECLS