staging: iio: ring_sw Add select of triggers to avoid build issue.

Currently all drivers that use ring_sw use triggers and I am yet
to see a good reason for any driver not doing so.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>