Fix condition evaluation on ansible_all_ipv6_addresses to have a Boolean result

The condition `when: ansible_all_ipv6_addresses` evaluates to a list, which in
term evaluates to True,  but it seems that more recent Ansible versions do not
accept that anymore:

```
Conditional result was "['fe80::...']" of type 'list', which evaluates to True. Conditionals must have a boolean result.
```

This commit adds a length check to create a real Boolean result.

Signed-off-by: Stefan Haun <mail@tuxathome.de>

roles/prereq/tasks/main.yml

@@ -24,7 +24,7 @@
     value: "1"
     state: present
     reload: true
-  when: ansible_all_ipv6_addresses
+  when: ansible_all_ipv6_addresses | length > 0
 
 - name: Populate service facts
   ansible.builtin.service_facts: