Start hiding "private/bionic_time.h".

Bug: 15765976

(cherry picked from commit 905e6d58aaf515bf237e62078f8a321920fe4c6b)

Change-Id: Ic7435308e8b521886f42ac7bf59531924921ea67
diff --git a/libc/include/sys/cdefs_elf.h b/libc/include/sys/cdefs_elf.h
index b5db13e..4dd7dc3 100644
--- a/libc/include/sys/cdefs_elf.h
+++ b/libc/include/sys/cdefs_elf.h
@@ -68,7 +68,7 @@
 #ifdef __LP64__
 #define __LIBC64_HIDDEN__ __LIBC_HIDDEN__
 #else
-#define __LIBC64_HIDDEN__
+#define __LIBC64_HIDDEN__ __LIBC_ABI_PUBLIC__
 #endif
 
 /* Used to tag non-static symbols that are public and exposed by the shared library. */