go/types: escape +build sequence to silence vet warning

Appease the vet builder until we figure out the ideal solution.

Updates #21716.

Change-Id: I64ecdf9b6efb0933c5c79e11c860966ff1980305
Reviewed-on: https://go-review.googlesource.com/60776
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/go/types/hilbert_test.go

@@ -52,7 +52,7 @@ 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
+// +`+`build ignore
 
 // This program tests arbitrary precision constant arithmetic
 // by generating the constant elements of a Hilbert matrix H,