[crypto] Use real prototypes for AXTLS' AES_encrypt() and AES_decrypt()

Avoid a compiler warning on some versions of gcc by using real
function prototypes.

Reported-by: Rob Shelley <Rob@cirris.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/crypto/axtls/os_port.h

 #define aes 1
 #if OBJECT
 
-/* AES_CTX is not defined at this point, so omit prototypes */
+struct aes_key_st;
 
-static void AES_encrypt();
-static void AES_decrypt();
+static void AES_encrypt ( const struct aes_key_st *ctx, uint32_t *data );
+static void AES_decrypt ( const struct aes_key_st *ctx, uint32_t *data );
 
 void axtls_aes_encrypt ( void *ctx, uint32_t *data ) {
 	AES_encrypt ( ctx, data );