Fix X11 build against FLTK 1.4.5 (explicit platform/X11 includes)

[wbniv]

Problem

Building the Linux (X11) target against FLTK 1.4.5 fails:

src/main-window.cpp: error: 'fl_display' was not declared in this scope
src/main-window.cpp: error: 'XA_CARDINAL' was not declared in this scope
src/main-window.h:  error: 'Pixmap' does not name a type

main-window.{cpp,h}, modal-dialog.cpp and option-dialogs.cpp use fl_display,
fl_xid, Pixmap and XA_CARDINAL under #ifndef _WIN32, but only <X11/xpm.h> is
included. The missing includes were introduced in
ea112f0
("Save and restore maximized window state in Linux (Xlib)") in 2021 — they were masked by
FLTK ≤ 1.4.4, which leaked these symbols transitively. FLTK 1.4.5 cleaned up its platform headers and stopped
leaking them, so the omission now causes a hard build failure.

Fix

Add the X11 / FLTK-platform includes to the shared src/utils.h (guarded #ifndef _WIN32),
so every translation unit that needs them gets them.

Build environment

Ubuntu 26.04, GCC 15, FLTK 1.4.5 built X11-only (FLTK_BACKEND_WAYLAND=0). Found while
packaging Tilemap Studio for Foundry Linux.

[wbniv]

more changes required for WAYLAND and/or X
bbs

[wbniv]

Closing this — the fix is already in upstream master.

The exact same change (#include <FL/platform.H> in main-window.h) was added by the maintainer in commit ffcce44 ("Update FLTK to 1.4.4", #87, 2025-09-12). That commit landed after the v4.0.1 tag, so anyone packaging the release tarball against FLTK 1.4.5 hits the build failure — but the fix is already in master, not here.

This PR was derived independently while packaging Tilemap Studio for Foundry Linux and happens to be the same single-line fix. No need to merge it.

For anyone packaging v4.0.1 against FLTK 1.4.5 and hitting this: cherry-pick ffcce44 or apply the patch manually — src/main-window.h, add #include <FL/platform.H> after the Fl_Native_File_Chooser.H line.