ubifs: Use correct config name for encryption

CONFIG_UBIFS_FS_ENCRYPTION is gone, fscrypt is now
controlled via CONFIG_FS_ENCRYPTION.
This problem slipped into the tree because of a mis-merge on
my side.

Reported-by: Eric Biggers <ebiggers@kernel.org>
Fixes: eea2c05d

 ("ubifs: Remove #ifdef around CONFIG_FS_ENCRYPTION")
Signed-off-by: Richard Weinberger <richard@nod.at>