s390/defconfigs: set CONFIG_NET_TC_SKB_EXT=y

As made explicit by commit 03a283cd ("net/mlx5: Kconfig: Make tc
offload depend on tc skb extension") tc skb extension is required for
offloading tc as well as bridges on switchdev capable ConnectX devices.

Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>