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/pthread.h b/libc/include/pthread.h
index d718b40..2fa10bb 100644
--- a/libc/include/pthread.h
+++ b/libc/include/pthread.h
@@ -33,11 +33,12 @@
  * @brief POSIX threads.
  */
 
+#include <sys/cdefs.h>
+
 #include <limits.h>
 #include <bits/page_size.h>
 #include <bits/pthread_types.h>
 #include <sched.h>
-#include <sys/cdefs.h>
 #include <sys/types.h>
 #include <time.h>