silence logrotate some more

I was getting email with logrotate error output from “which invoke-rc.d”
on systems without an invoke-rc.d.  This patch silences it.

Silence stderr from which when running logrotate

From: Alexandre Oliva <oliva@gnu.org>

Signed-off-by: Alexandre Oliva <oliva@gnu.org>
(cherry picked from commit d02340d90c9d30d44c962bea7171db3fe3bfba8e)