[FIX] website_forum: fix tag posts count dependencies

Post counts were not updated when a post was created or deleted
because of missing dependencies in the _compute.

Note that while "active_test" is included when counting without
specifying it, the field must be explicitly added to the `_compute`'s
dependency list to trigger it.

Task-3358143

closes odoo/odoo#124234

Signed-off-by: Stéphane Debauche (std) <std@odoo.com>