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
diff --git a/libc/stdio/fgetln.c b/libc/stdio/fgetln.c
index 946e1b7..95a5b31 100644
--- a/libc/stdio/fgetln.c
+++ b/libc/stdio/fgetln.c
@@ -43,7 +43,7 @@
  * so we add 1 here.
 #endif
  */
-int
+static int
 __slbexpand(FILE *fp, size_t newsize)
 {
 	void *p;