Move SCT_LIST_free definition into a more logical place

This reflects its position in include/openssl/ct.h.

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1548)
(cherry picked from commit e1940e9f)