add fio_set_directio() error message for platforms without direct I/O

This lets platforms without dio (e.g. OpenBSD) fail with an explicit
message, otherwise they fail without a clue about dio even with
--debug=all enabled.

The message modified based on comments from Jens.

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>