build: Install gtkboxlayout.h
This is included by gtk.h, and thus required to build.
From building GNOME To Do:
In file included from ../src/interfaces/gtd-activatable.h:24,
from ../src/gtd-window.c:23:
/usr/include/gtk-4.0/gtk/gtk.h:51:10: fatal error: gtk/gtkboxlayout.h: No such file or directory
#include <gtk/gtkboxlayout.h>