spice-client-glib-usb-acl-helper doesn't need suid root

A privileged executable should operate with the least amount of
privileges possible. For spice-client-glib-usb-acl-helper, this is
CAP_FOWNER for acl_set_file() to succeed.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Frediano Ziglio <fziglio@redhat.com>