staging/lustre: add BLOCK depends in Kconfig

Add BLOCK depends in Kconfig for LUSTRE to fix this:
drivers/staging/lustre/lustre/fid/../include/linux/lustre_compat25.h:117:2:
error: implicit declaration of function ‘unregister_blkdev’

Signed-off-by: Xiong Zhou <jencce.kernel@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>