pptp: Constify the po parameter of pptp_route_output().

Make it explicit that this function doesn't modify the socket passed as
parameter.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>