cmd/go: fix build

This was missed in c3b45d0dc5c0

R=golang-dev
CC=golang-dev
https://golang.org/cl/12379043

src/cmd/go/bootstrap.go

@@ -25,6 +25,6 @@ func httpsOrHTTP(importPath string) (string, io.ReadCloser, error) {
 	return "", nil, errHTTP
 }
 
-func parseMetaGoImports(r io.Reader) (imports []metaImport) {
+func parseMetaGoImports(r io.Reader) ([]metaImport, error) {
 	panic("unreachable")
 }