ECX_KEY was not meant for public consumption, it was only to be
accessed indirectly via EVP routines. However, we still need internal
access for our decoders.
This partially reverts 7c664b1f1bFixes#12880
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/12956)