xen/blkback: Fix compile warning

drivers/block/xen-blkback/xenbus.c:260:5: warning: symbol 'xenvbd_sysfs_addif' was not declared. Should it be static?
drivers/block/xen-blkback/xenbus.c:284:6: warning: symbol 'xenvbd_sysfs_delif' was not declared. Should it be static?

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>