curl_ntlm_core.c: Fixed compilation warning from commit 1cb17b2a5d

curl_ntlm_core.c:146: warning: passing 'DES_cblock' (aka 'unsigned char
                      [8]') to parameter of type 'char *' converts
                      between pointers to integer types with different
                      sign

lib/curl_ntlm_core.c

@@ -143,7 +143,7 @@ static void setup_des_key(const unsigned char *key_56,
 {
   DES_cblock key;
 
-  extend_key_56_to_64(key_56, key);
+  extend_key_56_to_64(key_56, (char *) key);
   DES_set_odd_parity(&key);
   DES_set_key(&key, ks);
 }