staging/lustre/obdclass: add empty tailing member to ctl_table

This was mistakenly removed by commit 7fb3d1c5.
Now register_sysctl_table() complains about "No proc_handler".

Cc: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Peng Tao <bergwolf@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>