go/types: add +build ignore to file generated by hilbert test

go/types: add +build ignore to file generated by hilbert test

This makes sure that the go/types package still builds even if
the hilbert test generated its test file in the go/types package
(when run as: go test -run Hilbert -out=h.go).

Change-Id: I60ecbaaa1537de14cfa95e2e6fc8ebedff651baf
Reviewed-on: https://go-review.googlesource.com/60531
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>