fix which-key--show-keymap for which-key-enable-extended-define-key

fix which-key--show-keymap for which-key-enable-extended-define-key

With which-key-enable-extended-define-key set to 't, given the keymap
and bindings below:

  (setq a-map (make-sparse-keymap))
  (bind-keys :map a-map
             ("a" . ("key 1" . command-1))
             ("b" . ("key b" . command-2)))

The output of (which-key--get-keymap-bindings a-map) was having
"unknown" as the key descriptions.

This change fixes it therefore allows the following to work as
expected:

(which-key--show-keymap "A map" a-map)