dm: fix DM_EVENT dependencies

CONFIG_DM_EVENT without CONFIG_EVENT is non-functional.
Let CONFIG_DM_EVENT depend on CONFIG_EVENT.

Remove superfluous stub in include/event.h.

Fixes: 5b896ed5 ("event: Add events for device probe/remove")
Reported-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>