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>