[FIX] stock: `report_stock_quantity` ambiguous column product_qty

In case there is a `product_qty` column added by a custom module,
`product_qty`, without specifying from which table to take it from
in the view definition, can lead to an ambiguous definition.

```
2021-08-25 12:25:31,624 1145 ERROR db_23001 odoo.modules.registry: Failed to load registry
Traceback (most recent call last):
  File "/home/odoo/src/odoo/13.0/odoo/modules/registry.py", line 86, in new
    odoo.modules.load_modules(registry._db, force_demo, status, update_module)
  File "/home/odoo/src/odoo/13.0/odoo/modules/loading.py", line 424, in load_modules
    force, status, report, loaded_modules, update_module, models_to_check)
  File "/home/odoo/src/odoo/13.0/odoo/modules/loading.py", line 315, in load_marked_modules
    perform_checks=perform_checks, models_to_check=models_to_check
  File "/home/odoo/src/odoo/13.0/odoo/modules/loading.py", line 202, in load_module_graph
    registry.init_models(cr, model_names, {'module': package.name}, new_install)
  File "/home/odoo/src/odoo/13.0/odoo/modules/registry.py", line 370, in init_models
    model.init()
  File "/home/odoo/src/odoo/13.0/addons/stock/report/report_stock_quantity.py", line 126, in init
    self.env.cr.execute(query)
  File "/home/odoo/src/odoo/13.0/odoo/sql_db.py", line 173, in wrapper
    return f(self, *args, **kwargs)
  File "/home/odoo/src/odoo/13.0/odoo/sql_db.py", line 250, in execute
    res = self._obj.execute(query, params)
psycopg2.errors.AmbiguousColumn: column reference "product_qty" is ambiguous
LINE 21: ...AND whd.id IS NULL) OR ls.usage = 'transit' THEN -product_qt...
```

upg-23001

closes odoo/odoo#76020

X-original-commit: 07f0ffad
Signed-off-by: Denis Ledoux (dle) <dle@odoo.com>