xen/privcmd: make option visible in Kconfig

xen/privcmd: make option visible in Kconfig

This configuration option provides a misc device as an API to userspace.
Make this API usable without having to select the module as a transitive
dependency.

This also fixes an issue where localyesconfig would select
CONFIG_XEN_PRIVCMD=m because it was not visible and defaulted to
building as module.

[boris: clarified help message per Jan's suggestion]

Based-on-patch-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20211116143323.18866-1-jgross@suse.com
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>