UI: Only execute "What's New" code on win32

The "What's New" dialog currently is only functional on windows.

UI/window-basic-main.cpp

@@ -1839,7 +1839,7 @@ void OBSBasic::OnFirstLoad()
 	if (api)
 		api->on_event(OBS_FRONTEND_EVENT_FINISHED_LOADING);
 
-#ifdef BROWSER_AVAILABLE
+#if defined(BROWSER_AVAILABLE) && defined(_WIN32)
 	/* Attempt to load init screen if available */
 	if (cef) {
 		WhatsNewInfoThread *wnit = new WhatsNewInfoThread();