Fix location of github.com/mdlayher/eui64

[ClaudioAlbertin]

The github.com/mdlayher/eui64 package used in ./dhcpv6/dhcpv6.go has been relocated to github.com/mdlayher/netx/eui64. Without this fix, bender and any libraries that depend on it cannot easily be installed successfully.

[cgordon]

@ClaudioAlbertin It looks like something else has gone wrong with the go vet command. It may be that the Thrift generator has changed since the last time we ran these checks. I will take a look later this week.