Add missing argument in function declaration, C23
https://bugs.gentoo.org/944020
--- a/bg.h
+++ b/bg.h
@@ -9,7 +9,7 @@
 #include <glib.h>
 #include <gdk-pixbuf/gdk-pixbuf.h>
 
-void bg_init();
+void bg_init(Display *dpyn);
 void bg_rootbg_changed();
 GdkPixmap *bg_new_for_win(Window win);
 void modify_drawable(GdkDrawable *base, GdkGC *gc, guint32 tintcolor, gint alpha);