sasl_sspi: Populate domain from the realm in the challenge

Without this, SSPI based digest auth was broken.

Bug: https://github.com/bagder/curl/pull/141.patch

lib/curl_sspi.h

@@ -43,6 +43,10 @@
 CURLcode Curl_sspi_global_init(void);
 void Curl_sspi_global_cleanup(void);
 
+/* This is used to populate the domain in a SSPI identity structure */
+CURLcode Curl_override_sspi_http_realm(const char *chlg,
+                                       SEC_WINNT_AUTH_IDENTITY *identity);
+
 /* This is used to generate an SSPI identity structure */
 CURLcode Curl_create_sspi_identity(const char *userp, const char *passwdp,
                                    SEC_WINNT_AUTH_IDENTITY *identity);