nspawn: userns and unified cgroup: chown cgroup.events

When starting a container in a new user namespace, systemd-nspawn chowns
the cgroup knob files so they are usable by the container. But the
cgroup knob file "cgroup.events" was missing. This file exists when the
unified hierarchy is used.

src/nspawn/nspawn-cgroup.c

@@ -54,6 +54,7 @@ int chown_cgroup(pid_t pid, uid_t uid_shift) {
                        "tasks",
                        "notify_on_release",
                        "cgroup.procs",
+                       "cgroup.events",
                        "cgroup.clone_children",
                        "cgroup.controllers",
                        "cgroup.subtree_control",