Hide ECX_KEY again

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.

Hide ECX_KEY again

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 7c664b1f1b5f60bf896f5fdea5c08c401c541dfe

Fixes #12880

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/12956)

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