Fix the build with gtkmm 4:

https://github.com/transmission/transmission/issues/6392

Patch copied from upstream source repository:

https://github.com/transmission/transmission/commit/e116672b27b314d54514c96b1fa7aef1dee900b1

From e116672b27b314d54514c96b1fa7aef1dee900b1 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?C=C5=93ur?= <coeur@gmx.fr>
Date: Sun, 17 Dec 2023 16:37:35 +0100
Subject: [PATCH] fix: build error on  GTKMM-4 (#6393)

---
 gtk/OptionsDialog.cc | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/gtk/OptionsDialog.cc b/gtk/OptionsDialog.cc
index 08198540c10..8c28fc76f98 100644
--- a/gtk/OptionsDialog.cc
+++ b/gtk/OptionsDialog.cc
@@ -338,7 +338,16 @@ void TorrentFileChooserDialog::onOpenDialogResponse(int response, Glib::RefPtr<S
         bool const do_prompt = get_choice(std::string(ShowOptionsDialogChoice)) == "true";
         bool const do_notify = false;
 
-        auto const files = IF_GTKMM4(get_files2, get_files)();
+#if GTKMM_CHECK_VERSION(4, 0, 0)
+        auto files = std::vector<Glib::RefPtr<Gio::File>>();
+        auto files_model = get_files();
+        for (auto i = guint{ 0 }; i < files_model->get_n_items(); ++i)
+        {
+            files.push_back(gtr_ptr_dynamic_cast<Gio::File>(files_model->get_object(i)));
+        }
+#else
+        auto const files = get_files();
+#endif
         g_assert(!files.empty());
 
         /* remember this folder the next time we use this dialog */