ruby: correctly set native/target dependencies

In particular libffi was missing from native, which
led to linking with host libffi instead.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 293c9f87)
Signed-off-by: Steve Sakoman <steve@sakoman.com>