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/mntent.h b/libc/include/mntent.h
index 9a31838..b4d1233 100644
--- a/libc/include/mntent.h
+++ b/libc/include/mntent.h
@@ -29,8 +29,9 @@
 #ifndef _MNTENT_H_
 #define _MNTENT_H_
 
-#include <stdio.h>
 #include <sys/cdefs.h>
+
+#include <stdio.h>
 #include <paths.h>  /* for _PATH_MOUNTED */
 
 #define MOUNTED _PATH_MOUNTED