netinet/in.h: Add definition for IPPROTO_GRE.

Commit b7ea2d480338(Extend OVS IPFIX exporter to export tunnel headers)
added a usage for IPROTO_GRE. But that does not look available in any
Visual Studio headers. So add it.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>

include/windows/netinet/in.h

@@ -0,0 +1,22 @@
+/*
+ * Copyright (c) 2014 Nicira, Inc.
+ *
+ * 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.
+ */
+
+#ifndef __NETINET_IN_H
+#define __NETINET_IN_H 1
+
+#define IPPROTO_GRE 47
+
+#endif /* netinet/in.h */