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/err.h b/libc/include/err.h
index d8122d7..4a1841b 100644
--- a/libc/include/err.h
+++ b/libc/include/err.h
@@ -36,8 +36,9 @@
  * @brief BSD error reporting functions. See `<error.h>` for the GNU equivalent.
  */
 
-#include <stdarg.h>
 #include <sys/cdefs.h>
+
+#include <stdarg.h>
 #include <sys/types.h>
 
 __BEGIN_DECLS