Hide __atexit and remove __atexit_invalid.

Bug: 11156955
Change-Id: I8c72edee8ecb92b75a282384277253bae19f7455
diff --git a/libc/stdlib/atexit.h b/libc/stdlib/atexit.h
index 2e88ad6..b530ade 100644
--- a/libc/stdlib/atexit.h
+++ b/libc/stdlib/atexit.h
@@ -45,8 +45,7 @@
 
 __BEGIN_DECLS
 
-extern int __atexit_invalid;
-extern struct atexit *__atexit;		/* points to head of LIFO stack */
+__LIBC_HIDDEN__ extern struct atexit *__atexit;		/* points to head of LIFO stack */
 
 int	__cxa_atexit(void (*)(void *), void *, void *);
 void	__cxa_finalize(void *);