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>
diff --git a/drivers/crypto/nx/nx.h b/drivers/crypto/nx/nx.h
index 591632d..9347878 100644
--- a/drivers/crypto/nx/nx.h
+++ b/drivers/crypto/nx/nx.h
@@ -149,6 +149,8 @@
 	} priv;
 };
 
+struct crypto_aead;
+
 /* prototypes */
 int nx_crypto_ctx_aes_ccm_init(struct crypto_aead *tfm);
 int nx_crypto_ctx_aes_gcm_init(struct crypto_aead *tfm);