vulkan-validation-layers: fix Darwin build by enabling robin-hood-hashing

pkgs/by-name/vu/vulkan-validation-layers/robin-hood-hashing.nix

···
   meta = with lib; {
     description = "Faster, more efficient replacement for std::unordered_map / std::unordered_set";
     homepage = "https://github.com/martinus/robin-hood-hashing";
-    platforms = platforms.linux;
+    platforms = platforms.unix;
     license = licenses.mit;
     maintainers = [ ];
   };