[FIX] mrp: compute the right producible qty with same components

Steps to reproduce:
- In Manufacturing, create a the following BoM
  - Product: P1
  - Components:
    - 2 x P2
    - 2 x P2
- Set the quantity in stock for P2 to 2
- Open the Overview from the BoM

Issue:
The 'Ready to Produce' column will indicate 1, as it checks line by line
how much product there is in stock compared to what is required for this
line.
To avoid this, we just need to sum the quantities required by products.

closes odoo/odoo#134714

Signed-off-by: William Henrotin (whe) <whe@odoo.com>