gpg: Allow generating Ed25519 key from an existing key.

* g10/misc.c (map_pk_gcry_to_openpgp): Add EdDSA mapping.

Due to this missing mapping a "gpg --export --full-gen-key" with
selection "13 - Existing key" did not worked for an ed25519 key.

• Signed-off-by: Werner Koch <wk@gnupg.org>