speed.c: use size_t instead of int to match function signatures

Signed-off-by: Cristian Stoica <cristian.stoica@nxp.com>

Reviewed-by: Emilia Käsper <emilia@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1378)