runtime/debug: fix build (missing Makefile)

Why does this happen so often?

R=rsc
CC=golang-dev
https://golang.org/cl/4067042

src/pkg/runtime/debug/Makefile

+# Copyright 2011 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.
+
+include ../../../Make.inc
+
+TARG=runtime/debug
+GOFILES=\
+	stack.go\
+
+include ../../../Make.pkg