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/60531Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

src/go/types/hilbert_test.go

@@ -52,6 +52,8 @@ func program(n int, out string) []byte {
 
 	g.p(`// Code generated by: go test -run=Hilbert -H=%d -out=%q. DO NOT EDIT.
 
+// +build ignore
+
 // This program tests arbitrary precision constant arithmetic
 // by generating the constant elements of a Hilbert matrix H,
 // its inverse I, and the product P = H*I. The product should