Win32: Add an include guard.

Add "#ifdef _WIN32" to file_dlg_win32.c so that it's ignored by
tools/validate-clang-check.sh.


(cherry picked from commit 6e1142c33a)

ui/win32/file_dlg_win32.c

@@ -8,6 +8,8 @@
  * SPDX-License-Identifier: GPL-2.0-or-later
  */
 
+#ifdef _WIN32
+
 #include "config.h"
 
 #include <tchar.h>
@@ -1796,6 +1798,8 @@ export_file_hook_proc(HWND ef_hwnd, UINT msg, WPARAM w_param, LPARAM l_param) {
     return 0;
 }
 
+#endif // _WIN32
+
 /*
  * Editor modelines
  *