[FIX] Fix for Warning "The domain term '('user_id', '=', [#])' should use the 'in' or 'not in' operator."

Executing the report 'Timesheet Profit' from the menu Reports->Reports->Timesheet Profit, Odoo raises a warning. 
This warning is due to the use of the '=' operator (as filter inside a 'search' function) to compare the field 'user_id' to a list of ids and not to a single id.