Add missing when statement

roles/matrix-server/tasks/start.yml

@@ -28,7 +28,7 @@
 
 - name: Ensure matrix-nginx-proxy autoruns and is restarted
   service: name=matrix-nginx-proxy enabled=yes state=restarted daemon_reload=yes
-  when:
+  when: matrix_nginx_proxy_enabled
 
 - name: Ensure matrix-corporal autoruns and is restarted
   service: name=matrix-corporal enabled=yes state=restarted daemon_reload=yes