Separate out disk available for Windows

Signed-off-by: Derek Collison <derek@nats.io>

server/disk_avail_windows.go

@@ -0,0 +1,21 @@
+// Copyright 2020 The NATS Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build windows
+
+package server
+
+// TODO(dlc) - See if there is a version of this for windows.
+func diskAvailable(storeDir string) int64 {
+	return JetStreamMaxStoreDefault
+}