lwtunnel: export linux/lwtunnel.h to userspace

Note also that include/linux/lwtunnel.h is not needed.

CC: Thomas Graf <tgraf@suug.ch>
CC: Roopa Prabhu <roopa@cumulusnetworks.com>
Fixes: 499a2425 ("lwtunnel: infrastructure for handling light weight tunnels like mpls")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>