cmd/godoc: add golang.org/x/build to go-import redirect list

Change-Id: Id372b2da716b31bc016855d2d7f783fd1764e5c0
Reviewed-on: https://go-review.googlesource.com/3110
Reviewed-by: Andrew Gerrand <adg@golang.org>

cmd/godoc/x.go

@@ -26,6 +26,7 @@ var xMap = map[string]xRepo{
 
 	"benchmarks": {"https://go.googlesource.com/benchmarks", "git"},
 	"blog":       {"https://go.googlesource.com/blog", "git"},
+	"build":      {"https://go.googlesource.com/build", "git"},
 	"crypto":     {"https://go.googlesource.com/crypto", "git"},
 	"debug":      {"https://go.googlesource.com/debug", "git"},
 	"exp":        {"https://go.googlesource.com/exp", "git"},