core: Set SECRET flag of subkeys stored on smart cards

* src/keylist.c (parse_sec_field15): Set SECRET flag.

Explicitly setting the SECRET flag for subkeys stored on smart cards
makes sure that the flag is also set if GPGME_KEYLIST_MODE_WITH_SECRET
is used when listing OpenPGP keys (as documented in the API
documentation). If secret keys are listed, then the flag is already set
when the packet type is parsed.

• GnuPG-bug-id: T5965