Product field already exists in the Report, all you need to do just select it by clicking on '+' towards left Total (i.e in default view, assuming measures are in Top)

Apart from that if you still need to add the field, then you will have to add in 2 places,one will be ofcourse in object 'account.invoice.report' and other will be in the view query which renders this object.

Once done, drop the view in PostgreSQL, and upgrade the module, which in turn will recreate View-Table for the object account.invoice.report.

Be noted, Object (which represents View) execution slightly varies from Other Object, infact the Data Insertion happens at 2 stages, either at Initial or when any action is taken place on its associated objects.