raid5-cache: add crc32c Kconfig dependency

The recent change of the raid5-cache code to use crc32c instead
of crc32 causes link errors when CONFIG_LIBCRC32C is disabled:

drivers/built-in.o: In function crc32c'
core.c:(.text+0x1c6060): undefined reference to `crc32c'

This adds an explicit 'select' statement like all other users
of this function do.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 5cb2fbd6ea0d ("raid5-cache: use crc32c checksum")
Signed-off-by: NeilBrown <neilb@suse.com>

drivers/md/Kconfig

@@ -123,6 +123,7 @@ config MD_RAID456
 	tristate "RAID-4/RAID-5/RAID-6 mode"
 	depends on BLK_DEV_MD
 	select RAID6_PQ
+	select LIBCRC32C
 	select ASYNC_MEMCPY
 	select ASYNC_XOR
 	select ASYNC_PQ