fscrypt: move fscrypt_ctx declaration to fscrypt_supp.h

Filesystems only ever access 'struct fscrypt_ctx' through fscrypt
functions.  But when a filesystem is built without encryption support,
these functions are all stubbed out, so the declaration of fscrypt_ctx
is unneeded.  Therefore, move it from fscrypt.h to fscrypt_supp.h.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>