kernel/timeout: Schedule zero-time timeouts

Fix an edge case that snuck in with the recent fix: if timeslicing is
enabled, the CPU's slice_ticks will be zero, and thus match a timeout
object's dticks value of zero, and thus get suppressed (because "we
already have a timeout scheduled for that") incorrectly.

Fixes #31789

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>