Fix no-posix-io

'openssl pkeyutl' uses stat() to determine the file size when signing using
Ed25519/Ed448, and this was guarded with OPENSSL_NO_POSIX_IO.

It is however arguable if stat() is a POSIX IO function, considering
that it doesn't use file descriptors, and even more so since we use
stat() elsewhere without that guard.

This will allow test/recipes/20-test_pkeyutl.t to be able to do its
work for Ed25519/Ed448 signature tests.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8498)