[fix] do not spit out error messages for utils/lxc.sh --help

Error::

  $ utils/lxc.sh --help
  ERROR: missing command lxc
  ...

This breaks also docs build::

  $ make docs
  ...
  WARNING: Unexpected return code 42 from command '../utils/lxc.sh --help'
  ...

Signed-off-by: Markus Heiser <markus@darmarit.de>