kdf: Avoid endianess issues

The key_type is only a letter, if we use and `int` and then cast it to
(const char *) we will end up with a 0 value on big endian.

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>
Reviewed-by: Anderson Toshiyuki Sasaki <ansasaki@redhat.com>

include/libssh/wrapper.h

@@ -103,7 +103,7 @@ size_t hmac_digest_len(enum ssh_hmac_e type);
 
 int ssh_kdf(struct ssh_crypto_struct *crypto,
             unsigned char *key, size_t key_len,
-            int key_type, unsigned char *output,
+            uint8_t key_type, unsigned char *output,
             size_t requested_len);
 
 int crypt_set_algorithms_client(ssh_session session);