mlxsw: spectrum_ipip: Add a forgotten include

struct ip_tunnel_parm, which is used in spectrum_ipip.h, is defined in
if_tunnel.h. However, the former neglects to include the latter.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>