ubifs: add CONFIG_BLOCK dependency for encryption

This came up during the v4.10 merge window:

warning: (UBIFS_FS_ENCRYPTION) selects FS_ENCRYPTION which has unmet direct dependencies (BLOCK)
fs/crypto/crypto.c: In function 'fscrypt_zeroout_range':
fs/crypto/crypto.c:355:9: error: implicit declaration of function 'bio_alloc';did you mean 'd_alloc'? [-Werror=implicit-function-declaration]
   bio = bio_alloc(GFP_NOWAIT, 1);

The easiest way out is to limit UBIFS_FS_ENCRYPTION to configurations
that also enable BLOCK.

Fixes: d475a507 ("ubifs: Add skeleton for fscrypto")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Richard Weinberger <richard@nod.at>