patch 8.1.2331: the option.c file is still very big

Problem:    The option.c file is still very big.
Solution:   Move a few functions to where they fit better. (Yegappan
            Lakshmanan, closes #4895)
diff --git a/src/proto/gui.pro b/src/proto/gui.pro
index a49ecad..7f2ce90 100644
--- a/src/proto/gui.pro
+++ b/src/proto/gui.pro
@@ -49,6 +49,8 @@
 void gui_check_colors(void);
 guicolor_T gui_get_color(char_u *name);
 int gui_get_lightness(guicolor_T pixel);
+char_u *gui_bg_default(void);
+void init_gui_options(void);
 void gui_new_scrollbar_colors(void);
 void gui_focus_change(int in_focus);
 void gui_mouse_moved(int x, int y);