Staging: lustre: obdclass: genops: Declare as static

Declare obd_export_nid2str and obd_zombie_impexp_cull as static
since they are used only in this particular file. Also remove the
corresponding declarations from header file

Signed-off-by: Shraddha Barke <shraddha.6596@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>