generic: add openssh-sftp-server

* adds ~800 KB binary size to ath79-generic because it pulls in openssl
* since OpenSSH 9.0 the old scp protocol is finally deprecated. To use
  that ancient protocol OpenSSH scp client needs a special parameter or
  a sftp server on the server side.

targets/generic

@@ -95,4 +95,5 @@ packages {
 	'-firewall4',
 	'gluon-core',
 	'ip6tables-legacy',
+	'openssh-sftp-server',
 }