crypto: nx - Add forward declaration for struct crypto_aead

The file nx.h has function prototypes that use struct crypto_aead.
However, as crypto/aead.h is not included we don't have a definition
for it.  This patch adds a forward declaration to fix this.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>