net/kni: remove driver struct forward declaration

net/kni: remove driver struct forward declaration

This patch removes the forward declaration of eth_kni_drv
in rte_eth_kni.c; this forward declaration was made unnecessary
by commit 050fe6e9ff970ff92d842912136be8f9f52e171f
("drivers/net: use ethdev allocation helper for vdev"), which
removes the usage of eth_kni_drv in the eth_kni_create() method.

Fixes: 050fe6e9ff97 ("drivers/net: use ethdev allocation helper for vdev")
Cc: stable@dpdk.org
Signed-off-by: Rami Rosen <rami.rosen@intel.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>