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/sys/xattr.h b/libc/include/sys/xattr.h
index 38c11e2..ebe4eb8 100644
--- a/libc/include/sys/xattr.h
+++ b/libc/include/sys/xattr.h
@@ -33,8 +33,9 @@
  * @brief Extended attribute functions.
  */
 
-#include <linux/xattr.h>
 #include <sys/cdefs.h>
+
+#include <linux/xattr.h>
 #include <sys/types.h>
 
 __BEGIN_DECLS