Always include <sys/cdefs.h> first.

Otherwise you can't be sure that things like __BIONIC__ work.

Change-Id: Ic5e59988b69419828c3b1a97332c7e2ab58e09a1
diff --git a/libc/include/sched.h b/libc/include/sched.h
index e8f7736..7a2dcad 100644
--- a/libc/include/sched.h
+++ b/libc/include/sched.h
@@ -33,9 +33,10 @@
  * @brief Thread execution scheduling.
  */
 
+#include <sys/cdefs.h>
+
 #include <bits/timespec.h>
 #include <linux/sched.h>
-#include <sys/cdefs.h>
 
 __BEGIN_DECLS