ruby: add PACKAGECONFIG for capstone

Autobuilder workers were non-deterministically enabling capstone
depending on whether the worker had libcapstone installed.

Add PACKAGECONFIG for capstone with default off, since ruby does not
require capstone support.

Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>