fs: btrfs: Select SHA256 in Kconfig

Since commit 565a4147 ("fs: btrfs: Add more checksum algorithms")
btrfs uses the sha256 checksum algorithm. But Kconfig lacks to select
it. This leads to compilation errors:
fs/built-in.o: In function `hash_sha256':
fs/btrfs/crypto/hash.c:25: undefined reference to `sha256_starts'
fs/btrfs/crypto/hash.c:26: undefined reference to `sha256_update'
fs/btrfs/crypto/hash.c:27: undefined reference to `sha256_finish'

Signed-off-by: Matthias Brugger <mbrugger@suse.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>