go.tools/ssa: fix build

Caused by https://golang.org/cl/11093044/ .

R=adonovan
CC=golang-dev
https://golang.org/cl/11149043

ssa/builder_test.go

@@ -69,9 +69,9 @@ func main() {
 		isExt := pkg != mainPkg
 
 		// init()
-		if isExt && !isEmpty(pkg.init) {
+		if isExt && !isEmpty(pkg.Func("init")) {
 			t.Errorf("external package %s has non-empty init", pkg)
-		} else if !isExt && isEmpty(pkg.init) {
+		} else if !isExt && isEmpty(pkg.Func("init")) {
 			t.Errorf("main package %s has empty init", pkg)
 		}
 

ssa/example_test.go

@@ -73,6 +73,7 @@ func main() {
 	// Output:
 	//
 	// package main:
+	//   func  init       func()
 	//   var   init$guard *bool
 	//   func  main       func()
 	//   const message    message = "Hello, World!":untyped string