cnp: init wayland dnd handlers upon creating the first drop target

previously this would only init when the app was initiating the drag,
which would result in an app being unable to receive drags

@fix

legacy/elementary/src/lib/elm_cnp.c

@@ -2754,6 +2754,7 @@ _wl_elm_drop_target_add(Evas_Object *obj, Elm_Sel_Format format, Elm_Drag_State
         handler_drop =
           ecore_event_handler_add(ECORE_WL_EVENT_DND_DROP,
                                   _wl_dnd_drop, NULL);
+        _wl_elm_dnd_init();
      }
 
    return EINA_TRUE;