Show workspace in _tree_next

Fixes a regression introduced by #2980.

src/tree.c

@@ -565,6 +565,7 @@ static bool _tree_next(Con *con, char way, orientation_t orientation, bool wrap)
             focus = con_descend_focused(workspace);
         }
 
+        workspace_show(workspace);
         con_focus(focus);
         x_set_warp_to(&(focus->rect));
         return true;