Fix possible fall-through in outliner region listener

Reported as a strict compiler warning, and the need of fall-through is
not needed from just reading the code.

If it is something what must happen, the reasoning is to be explained
in the comment, and ATTR_FALLTHROUGH is to be used.

source/blender/editors/space_outliner/space_outliner.c

@@ -118,6 +118,7 @@ static void outliner_main_region_listener(wmWindow *UNUSED(win),
           ED_region_tag_redraw(region);
           break;
       }
+      break;
     case NC_SCENE:
       switch (wmn->data) {
         case ND_OB_ACTIVE: