http_negotiate: Corrected host and proxy host name being wrong way round

I had accidentally used the proxy server name for the host and the host
server name for the proxy in commit ad5e9bfd and 6d6f9ca1. Whilst
Windows SSPI was quite happy with this, GSS-API wasn't.

Thanks-to:  Michael Osipov

lib/http_negotiate.c

@@ -54,14 +54,14 @@ CURLcode Curl_input_negotiate(struct connectdata *conn, bool proxy,
     userp = conn->proxyuser;
     passwdp = conn->proxypasswd;
     service = data->set.str[STRING_PROXY_SERVICE_NAME];
-    host = conn->host.name;
+    host = conn->proxy.name;
     neg_ctx = &data->state.proxyneg;
   }
   else {
     userp = conn->user;
     passwdp = conn->passwd;
     service = data->set.str[STRING_SERVICE_NAME];
-    host = conn->proxy.name;
+    host = conn->host.name;
     neg_ctx = &data->state.negotiate;
   }