Fix prototype warnings triggered by -Wstrict-prototypes when configuring
with `enable-ec_nistp_64_gcc_128`
Reviewed-by: NKurt Roeckx <kurt@roeckx.be>
Reviewed-by: NMatthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/6556)