openssh: Add openssh-sftp-server to openssh RDEPENDS

OpenSSH 9.0 uses sftp by default as the transport for scp, add in
sftp-server so that this works as expected for users, rather than being
left with a confusing "scp: Connection closed" message.

Signed-off-by: Alex Kiernan <alexk@zuma.ai>
Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>