agent: Fix ECC key on smartcard for composite KEM with PQC.
* agent/divert-scd.c (agent_card_ecc_kem): Convert binary keygrip to HEXto call agent_card_pkdecrypt. Handle the case with prefix.
- GnuPG-bug-id: T7648
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>