agent: Fix secret key import for Ed25519.
* agent/cvt-openpgp.c (convert_secret_key): Avoid adding 0x00 at the beginning of MPI.
In master, we handle it as opaque MPI, but in 2.2, we use standard MPI
here.
- GnuPG-bug-id: T5114
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>