musl: more stdio files from musl

libc/stdio/stdio_ext.h is identical to include/api/stdio_ext.h, so can
be removed. ext.c's only change was from <stdio_ext.h> to "stdio_ext.h"
so it can be removed too.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>