staging: lustre: fix sparse warning

Sparse reports:

drivers/staging/lustre/lustre/obdclass/obd_mount.c:1284:6: warning:
 symbol 'lustre_kill_super' was not declared. Should it be static?

Fix this warning by making lustre_kill_super static.
It is not used outside this file.

Signed-off-by: Tal Shorer <tal.shorer@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>