cmd/api: add missing runtime struct

Fixes build.

TBR=khr
R=golang-codereviews
CC=golang-codereviews, khr
https://golang.org/cl/130390044

src/cmd/api/goapi.go

@@ -381,7 +381,7 @@ func (w *Walker) parseFile(dir, file string) (*ast.File, error) {
 		src := "package runtime; type maptype struct{}; type _type struct{}; type alg struct{};" +
 			" type mspan struct{}; type m struct{}; type lock struct{}; type slicetype struct{};" +
 			" type iface struct{}; type eface struct{}; type interfacetype struct{}; type itab struct{};" +
-			" type mcache struct{}"
+			" type mcache struct{}; type bucket struct{}"
 		f, err = parser.ParseFile(fset, filename, src, 0)
 		if err != nil {
 			log.Fatalf("incorrect generated file: %s", err)