g10: Move enum_secret_keys to skclist.c.

* g10/getkey.c (enum_secret_keys): Move to...
* g10/skclist.c (enum_secret_keys): ... here.

The function enum_secret_keys is not used by gpgv.c, but it is in
getkey.c. Extending enum_secret_keys will require change of gpgv.c,
so moving the function to the file for gpg is better.

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