Fixed lib.fixpoint in do_lob

diaframe/lib/do_lob.v

-From iris.bi Require Export bi telescopes lib.laterable lib.fixpoint.
+From iris.bi Require Export bi telescopes lib.laterable lib.fixpoint_mono.
 From iris.proofmode Require Import proofmode environments.
 From diaframe Require Import proofmode_base utils_ltac2.