tunnel: add a copy of include/linux/if_tunnel.h

This should fix the following compile error:

route/link/ipip.c:52: error: ‘IFLA_IPTUN_MAX’ undeclared here (not in a function)
route/link/ipip.c:53: error: ‘IFLA_IPTUN_LINK’ undeclared here (not in a function)
route/link/ipip.c:53: error: array index in initializer not of integer type
route/link/ipip.c:53: error: (near initialization for ‘ipip_policy’)
route/link/ipip.c:53: error: field name not in record or union initializer
route/link/ipip.c:53: error: (near initialization for ‘ipip_policy’)
...

Reported-by: Vinson Lee <vlee@twitter.com>
Cc: Susant Sahani <susant@redhat.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Thomas Haller <thaller@redhat.com>