Params: add OSSL_PARAM_construct_end()

OSSL_PARAM_END is a macro that can only be used to initialize an
OSSL_PARAM array, not to assign an array element later on.  For
completion, we add an end constructor to facilitate that kind of
assignment.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8704)