g10: Fix another memory leak.

* g10/skclist.c (enum_secret_keys): Use SK_LIST instead of pubkey_t.

The use of pubkey_t was wrong. The use is just a list of keys, not
with keyblock. With SK_LIST, release_sk_list releases memory by
free_public_key.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>