Provide missing let definition to prevent background void-variable error.

* lisp/allout-widgets.el (allout-widgets-exposure-change-processor)
Let-declare handled-conceal, for reference through `(symbol-value)'
within the let body. (Because the error happens in an
after-change-functions hook, so it is caught and reported as a message
by allout-widgets-hook-error-handler.)

lisp/allout-widgets.el

@@ -988,6 +988,7 @@ Generally invoked via `allout-exposure-change-functions'."
         ;; have to distinguish between concealing and exposing so that, eg,
         ;; `allout-expose-topic's mix is handled properly.
         handled-expose
+        handled-conceal
         covered
         deactivate-mark)