Timesheet cost is not included on production cost

Steps to reproduce:

1. Install production_timesheet_cost
2. Create an employee a set an hourly cost of 30.00
3. Create a product with a work with an operation that accepts timesheet lines and a cost center with 0 cost price
4. Start the work and include a timesheet line of 1 hour.

Current behaviour:

• The cost of the work is zero

Excepted behaviour:

• The cost of the work is 30.00 (1hour x hourly rate).

Found on 7.0 series.