UI: Add HTTP header if the update check is manually initiated

With the addition of server-side release rate control, having the server
know if the update check was manually initiated can allow it to deliver
the update to the user even if they would normally not be eligible.
Windows only.

UI/win-update/win-update.cpp

@@ -585,6 +585,11 @@ try {
 		extraHeaders.push_back(move(header));
 	}
 
+	/* allow server to know if this was a manual update check in case
+	 * we want to allow people to bypass a configured rollout rate */
+	if (manualUpdate)
+		extraHeaders.emplace_back("X-OBS2-ManualUpdate: 1");
+
 	/* ----------------------------------- *
 	 * get manifest from server            */