UI: Fix compiling without nlohmann JSON

After commit 89553386247, 'UI/update/models/whatsnew.hpp' is included
unconditionally, even with 'ENABLE_WHATSNEW=OFF', which breaks
building OBS Studio without nlohmann JSON.

UI/window-basic-main.cpp

@@ -84,6 +84,10 @@
 #include "windows.h"
 #endif
 
+#ifdef WHATSNEW_ENABLED
+#include "update/models/whatsnew.hpp"
+#endif
+
 #if !defined(_WIN32) && defined(WHATSNEW_ENABLED)
 #include "update/shared-update.hpp"
 #endif
@@ -97,8 +101,6 @@
 
 #include <QWindow>
 
-#include "update/models/whatsnew.hpp"
-
 #ifdef ENABLE_WAYLAND
 #include <obs-nix-platform.h>
 #endif