config: fix zeitwekr warning when reloading the app

Turns out we need not only to load the Job constants later, but also
not to do the same work twice – otherwise we'll get a 

> ApiEntreprise::Job constant is already defined

when attempting to re-define the constant.

config/initializers/active_job_compatibility.rb

@@ -11,9 +11,10 @@
 # - but let's keep these for a while to make external integrators's life easier.
 # To keep some margin, let's say this file can be safely deleted in May 2021.)
 
-require 'excon'
-
 Rails.application.reloader.to_prepare do
+  if !defined?(ApiEntreprise)
+    require 'excon'
+
     module ApiEntreprise
       Job = APIEntreprise::Job
       AssociationJob = APIEntreprise::AssociationJob
@@ -29,4 +30,5 @@ Rails.application.reloader.to_prepare do
     module Cron
       FixMissingAntivirusAnalysis = FixMissingAntivirusAnalysisJob
     end
+  end
 end