link: link types are missing in gentypes

[Asphaltt]

By referencing bpftool link, there are two missing functions: LinkGetNextID() and NewLinkFromID().

Then, when I want to implement them, LinkGetNextIdAttr, LinkGetNextID() and LinkGetFDByID() are missing in sys/types.go generated by gentypes.

I'm not familiar with gentypes to add the missing attr and funcs.

As a result, I'm not able to list all links. Or, I have to hack link/link.go to implement the missing functions by clumsy way.