mm: zram: fix build error when zram wb disabled

Change-Id: I0183a3d5ad6b6cc89df3d7ff33bc771f5aca5708
Signed-off-by: huangzq2 <huangzq2@motorola.com>
Reviewed-on: https://gerrit.mot.com/1902411


SLTApproved: Slta Waiver
SME-Granted: SME Approvals Granted
Tested-by: Jira Key
Reviewed-by: Xiangpo Zhao <zhaoxp3@motorola.com>
Submit-Approved: Jira Key
Signed-off-by: UtsavBalar1231 <utsavbalar1231@gmail.com>