Make the GTK font setup error box non-modal.

That was the last (in fact, the only) call to the modal version of
message_box() outside gtkdlg.c, so I can remove it from the header
file.

unix/gtkdlg.c

@@ -1084,7 +1084,7 @@ static void set_transient_window_pos(GtkWidget *parent, GtkWidget *child)
 #endif
 }
 
-static void trivial_post_dialog_fn(void *vctx, int result)
+void trivial_post_dialog_fn(void *vctx, int result)
 {
 }