EVP_PKEY_size declared to take a const parameter

CLA: trivial
Function EVP_PKEY_size has been modified to take a const parameter

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/7892)