add missing const in cast
add missing const in cast

Reviewed-by: Richard Levitte <levitte@openssl.org>

Reviewed-by: Viktor Dukhovni <86faeb3e05561b856666236e198c27e698275e82@openssl.org>

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl