r/protonvpn: Install watchdog dependencies

The `protonvpn-watchdog.py` script requires the *httpx* Python package.
This needs to be installed for the script to work.

roles/protonvpn/tasks/main.yml

@@ -28,6 +28,12 @@
   - strongswan-config
   - protonvpn-config
 
+- name: ensure protonvpn-watchdog dependencies are installed
+  package:
+    name: python3-httpx
+    state: present
+  tags:
+  - install
 - name: ensure protonvpn-watchdog script is installed
   copy:
     src: protonvpn-watchdog.py