[FIX] stock_dropshipping: correct dependencies

`purchase_stock` is a required dependencies because the `stock.rule`
`stock_rule_drop_shipping` use `buy` as `action` [1].
However, it is added to the selection by the module `purchase_stock` [2].

[1] https://github.com/odoo/odoo/blob/c4b6273c9df5d8e3f3a5ee3cf52541741526280e/addons/stock_dropshipping/data/stock_data.xml#L32
[2] https://github.com/odoo/odoo/blob/c4b6273c9df5d8e3f3a5ee3cf52541741526280e/addons/purchase_stock/models/stock_rule.py#L14

closes odoo#43218

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>

addons/stock_dropshipping/__manifest__.py

@@ -20,7 +20,7 @@
 internal transfer document is needed.
 
 """,
-    'depends': ['sale_purchase', 'sale_stock'],
+    'depends': ['sale_purchase', 'sale_stock', 'purchase_stock'],
     'data': ['data/stock_data.xml', 'views/sale_order_views.xml'],
     'installable': True,
     'auto_install': False,