sch_cake: Add missing NLA policy entry TCA_CAKE_SPLIT_GSO

This field has never been checked since introduction in mainline kernel

Signed-off-by: Victorien Molle <victorien.molle@wifirst.fr>
Signed-off-by: Florent Fourcot <florent.fourcot@wifirst.fr>
Fixes: 2db6dc26 "sch_cake: Make gso-splitting configurable from userspace"
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>