libprocessgroup: Fix warnings when legacy schedtune cgroup is absent

Issue a warning about missing cpu/schedtune controller only if both of
them are missing.

Bug: 185437398
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: I3a9d3c9a8c91c8d2c5346bcb431bb0407c64a811