curl_ntlm_core.c: Fixed compilation warnings

curl_ntlm_core.c:301: warning: pointer targets in passing argument 2 of
                      'CryptImportKey' differ in signedness
curl_ntlm_core.c:310: warning: passing argument 6 of 'CryptEncrypt' from
                      incompatible pointer type
curl_ntlm_core.c:540: warning: passing argument 4 of 'CryptGetHashParam'
                      from incompatible pointer type