Fix missing lexbind declaration in slack-user-message.

[ieure]

Recent Emacs builds have changed how cl-call-next-method works in a way that causes breakage when used with dynamic binding (but they work fine with lexical binding). The slack-user-message.el file appears to have a missing / broken / incomplete header, which causes it to default to dynamic binding, which causes cl-call-next-method inside `slack-message-body' to fail.

This commit adds the file header and declares that slack-user-message should use lexical binding.

fixes yuya373/emacs-slack#570

[unhammer]

That fixes it for me, thanks =D