lightnvm: select CONFIG_CRC32

Without CRC32 support, this fails to link:

arm-linux-gnueabi-ld: drivers/lightnvm/pblk-init.o: in function `pblk_init':
pblk-init.c:(.text+0x2654): undefined reference to `crc32_le'
arm-linux-gnueabi-ld: drivers/lightnvm/pblk-init.o: in function `pblk_exit':
pblk-init.c:(.text+0x2a7c): undefined reference to `crc32_le'

Fixes: a4bd217b ("lightnvm: physical block device (pblk) target")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>