xen kconfig: fix select INPUT_XEN_KBDDEV_FRONTEND

A randconfig compile test discovered that we can select
INPUT_XEN_KBDDEV_FRONTEND without all of its dependencies being met. Fix
this by adding the dependency to the select line.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>