kernel: fix z_is_inactive_timeout stab when missing sys clock

Currently we provide incorrect stab value of z_is_inactive_timeout
in case of the configuration without system clock
(CONFIG_SYS_CLOCK_EXISTS=n). This prevents threads from being
scheduled (we don't even schedule main thread in hello world
example if CONFIG_SYS_CLOCK_EXISTS=n).

Fix that.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>