Include timesheet cost in production work only if timesheet cost module is activated

f2642fe60c20 extends the cost method using the cost price of the timesheet line
but without ensuring the module is activated.

Closes #14830
(grafted from 4b38a680f4ee2139a8d0179a3a67eb2606faa7e4)

--HG--
branch : 8.0

modules/production_work_timesheet/tryton.cfg

@@ -20,3 +20,4 @@ model:
 [register timesheet_cost]
 model:
     work.WorkCenter
+    work.Work_TimesheetCost

modules/production_work_timesheet/work.py

@@ -130,6 +130,10 @@ class Work(metaclass=PoolMeta):
                     'timesheet_end_date': date,
                     })
 
+
+class Work_TimesheetCost(metaclass=PoolMeta):
+    __name__ = 'production.work'
+
     @classmethod
     def _get_cost(cls, works):
         pool = Pool()