Suppress some warnings about un-prefixed dynamic variables

* lisp/mh-e/mh-mime.el (mh-insert-mime-security-button): Suppress
warnings about un-prefixed dynamic variables.
(mh-insert-mime-button): Ditto.
1 job for master in 74 minutes and 44 seconds (queued for 7 seconds)
Status Job ID Name Coverage
  Test
passed #3333
test-all

01:14:44