UBI: block: Add CONFIG_BLOCK dependency

The UBI block driver depends on the block infrastructure. Add the
proper dependency and fix a build error when CONFIG_BLOCK is not selected.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>