musl: remove libc/stdio/stdio.h

Remove libc/stdio/stdio.h, which was an unnecessary slightly-modified copy
of include/api/stdio.h. Musl doesn't have a stdio/stdio.h file either.
In a few places that included "stdio.h" and similar, include <stdio.h> instead.

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