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/mktemp.c

@@ -65,7 +65,7 @@ mkdtemp(char *path)
 
 char *_mktemp(char *);
 
-char *
+__LIBC_HIDDEN__ char *
 _mktemp(char *path)
 {
 	return(_gettemp(path, (int *)NULL, 0, 0) ? path : (char *)NULL);