patch 8.1.2034: dark them of GTK 3 not supported

Problem:    Dark them of GTK 3 not supported.
Solution:   Add the "d" flag in 'guioptions'. (Jonathan Conder, closes #4934)
diff --git a/src/proto/gui_gtk_x11.pro b/src/proto/gui_gtk_x11.pro
index 708e68d..3f26044 100644
--- a/src/proto/gui_gtk_x11.pro
+++ b/src/proto/gui_gtk_x11.pro
@@ -8,6 +8,7 @@
 void gui_mch_start_blink(void);
 int gui_mch_early_init_check(int give_message);
 int gui_mch_init_check(void);
+void gui_mch_set_dark_theme(int dark);
 void gui_mch_show_tabline(int showit);
 int gui_mch_showing_tabline(void);
 void gui_mch_update_tabline(void);