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

drivers/crypto/nx/nx.h

@@ -149,6 +149,8 @@ struct nx_crypto_ctx {
 	} 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);