x11-toolkits/gtk40: unbreak consumers after a5d572b40b8b

In file included from /usr/local/include/gtk-4.0/gtk/gtk.h:31:
In file included from /usr/local/include/gtk-4.0/gdk/gdk.h:76:
/usr/local/include/gtk-4.0/gdk/gdkvulkancontext.h:32:10: fatal error: 'vulkan/vulkan.h' file not found
#include <vulkan/vulkan.h>

^~~~~~~~~~~~~~~~~

PR: 256344
Reported by: John Hein <jcfyecrayz@liamekaens.com>