refactor: Default kscan events for native posix

* By default, don't generate any events for the native
  posix board DTS, to allow native builds outside
  of the testing use case.

app/boards/native_posix_64.overlay

@@ -14,5 +14,6 @@
         rows = <2>;
         columns = <2>;
         exit-after;
+        events = <>;
     };
 };