zram: fix unused 'zram_wb_devops' warning

zram: fix unused 'zram_wb_devops' warning

drivers/block/zram/zram_drv.c:55:45: warning: 'zram_wb_devops' defined but not used [-Wunused-const-variable=]

Fix the above warning if CONFIG_ZRAM_WRITEBACK not enabled.

Link: https://lkml.kernel.org/r/20220608072534.68850-1-wangkefeng.wang@huawei.com
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>