testutil/driver.c: Fix function prototype warning [-Wstrict-prototypes]

(introduced by commit 91860165, which added -Wstrict-prototypes)

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
(Merged from https://github.com/openssl/openssl/pull/6555)