ovs-router: Fix build on NetBSD

Split the linux-specific part of ovs-router.h
into ovs-router-linux.h.

The breakage was introduced by
commit d9b4ebc5d15202bedad06969353435a4c1951c71
("route-table: Use classifier to store routing table.")

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Acked-by: Ben Pfaff <blp@nicira.com>