virtualbox: expose extensionPack in meta

This way it can be added to system.extraDependencies to save it from gc

pkgs/applications/virtualization/virtualbox/default.nix

···
     homepage = http://www.virtualbox.org/;
     maintainers = [ lib.maintainers.sander ];
     platforms = lib.platforms.linux;
+    inherit extensionPack;
   };
 }