stating/lustre: only build if configured as module

Lustre internal dependency needs to be cleaned up. Currently,
libcfs is acting as a basis of all other modules, while other
modules in lustre/ directory in turn depend on lnet modules.
It creates a dependency loop that need to be fixed. Hopefully
we will remove libcfs in the end. So just disable buildin for
now.

Signed-off-by: Peng Tao <tao.peng@emc.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>