The OpenSSL engine passes a "const u_char *" to the callback but

The OpenSSL engine passes a "const u_char *" to the callback but
relayd's RSA privsep engine uses an iovec that expects a non-cast
"void *".  Cast it and disable the -Wcast-qual warning because I don't
want to copy the data and I didn't find a better way to implement it.

ok guenther@