cmd/cgo: fix build (missing file from earlier CL)

TBR=golang-dev
CC=golang-dev
https://golang.org/cl/13700045

misc/cgo/test/issue6390.go

+// Copyright 2013 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cgotest
+
+// #include <stdlib.h>
+import "C"
+
+import "testing"
+
+func test6390(t *testing.T) {
+	p1 := C.malloc(1024)
+	if p1 == nil {
+		t.Fatalf("C.malloc(1024) returned nil")
+	}
+	p2 := C.malloc(0)
+	if p2 == nil {
+		t.Fatalf("C.malloc(0) returned nil")
+	}
+	C.free(p1)
+	C.free(p2)
+}