[dev.cc] runtime: add missing cb_max const

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/169490043

src/runtime/runtime2_windows.go

+// Copyright 2009 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 runtime
+
+// TODO(brainman): move generation of zsys_windows_*.s out from cmd/dist/buildruntime.c and into here
+const cb_max = 2000 // maximum number of windows callbacks allowed (must be in sync with MAXWINCB from cmd/dist/buildruntime.c)