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

drivers/staging/lustre/lustre/Kconfig

 config LUSTRE_FS
 	tristate "Lustre file system client support"
-	depends on STAGING && INET && m
+	depends on STAGING && INET && BLOCK && m
 	select LNET
 	select CRYPTO
 	select CRYPTO_CRC32