fltk: improve compatibility with fltk 1.4.x

src/ui-fltk/main_window.cpp

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: GPL-3.0-or-later
-// SPDX-FileCopyrightText: 2024-2025 Ivan Baidakou
+// SPDX-FileCopyrightText: 2024-2026 Ivan Baidakou
 
 #include "main_window.h"
 
@@ -12,6 +12,7 @@
 
 #include <FL/Fl_Box.H>
 #include <FL/Fl_Tile.H>
+#include <FL/platform.H>
 #include <fmt/format.h>
 
 using namespace syncspirit;