x11-toolkits/gtk40: enable VULKAN by default
Vulkan has to be explicitly enabled at runtime e.g.,
$ GSK_RENDERER=vulkan gtk4-demoPR: 256333
Inspired by: Alpine, AOSC, Arch, Ataraxia, Debian, KaOS, Mageia, Nix, openmamba, OpenMandriva, openSUSE, Pisi, Void
Approved by: tcberner