stdio: make internal symbols static/hidden

Although header libc/stdio/local.h declares the macros and private
variables of stdio, there are several internal symbols exposed
unexpectedly.

Change-Id: Ie7a07f85b70322fb9cd05b3c8e1bcc416061eb4b

libc/stdio/flockfile.c

@@ -191,7 +191,7 @@ funlockfile(FILE * fp)
 
 
 /* called from fclose() to remove the file lock */
-void
+__LIBC_HIDDEN__ void
 __fremovelock(FILE*  fp)
 {
     LockTable*  t = lock_table_lock();