kernel: events: Depend on multithreading

Kernel events depend on multithreading being enabled, and mixing them
with a non-multithreaded build gives linker failures internal to
events.c. To avoid this, make events depend on multithreading.

```
libkernel.a(events.c.obj): in function `k_event_post_internal':
175: undefined reference to `z_sched_waitq_walk'
events.c:183: undefined reference to `z_sched_wake_thread'
events.c:191: undefined reference to `z_reschedule'
libkernel.a(events.c.obj): in function `k_sched_current_thread_query':
kernel.h:216: undefined reference to `z_impl_k_sched_current_thread_query'
libkernel.a(events.c.obj): in function `k_event_wait_internal':
events.c:312: undefined reference to `z_pend_curr'
```

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>

Author: carlescufi

kernel/Kconfig

@@ -734,6 +734,7 @@ config NUM_MBOX_ASYNC_MSGS
 
 config EVENTS
 	bool "Event objects"
+	depends on MULTITHREADING
 	help
 	  This option enables event objects. Threads may wait on event
 	  objects for specific events, but both threads and ISRs may deliver