zram.ko needs CONFIG_BLK_DEV activated and it is by default for all
other targets in OpenWrt.
This makes zram.ko compile again.
Compile tested only.
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit 6745af9a0de28171b45c0e8584e393bd80f0a377)