mu4e: fix compiler warnings

mu4e/mu4e-folders.el

@@ -115,7 +115,7 @@ shortcut (e.g., \"/archive\").
 Example:
 
  (setq mu4e-maildir-shortcuts
-       '((:maildir \"/inbox\"     :key ?i :hide-if-no-unread t)
+     \='((:maildir \"/inbox\"     :key ?i :hide-if-no-unread t)
          (:maildir \"/drafts\"    :key ?d :hide t)
          (:maildir \"/sent\"      :key ?s :hide-unread t)))
 

mu4e/mu4e-headers.el

@@ -1038,10 +1038,10 @@ you cannot sort by this field.
 If THREADS is non-nil, give a more informative error message."
   (if (not sortable)
       (if threads
-          (lambda (&optional e)
+          (lambda (&optional _e)
             (interactive "e")
             (mu4e-message "With threading, you can only sort by date"))
-        (lambda (&optional e)
+        (lambda (&optional  _e)
           (interactive "e")
           (mu4e-message "Field is not sortable")))
     (lambda (&optional e)