kni: fix build with Linux 5.1

kni: fix build with Linux 5.1

Build error seen with Linux kernel 5.1 and
when CONFIG_RTE_KNI_KMOD_ETHTOOL is enabled.

Build error:
kernel/linux/kni/igb_main.c:2352:18:
  error: initialization of ... from incompatible pointer type ...
  [-Werror=incompatible-pointer-types]
    .ndo_fdb_add  = igb_ndo_fdb_add,
                      ^~~~~~~~~~~~~~~

ndo_fdb_add() is changed in Linux kernel version 5.1 and now requires
a new parameter, 'struct netlink_ext_ack *extack':
Linux Commit 87b0984ebfab ("net: Add extack argument to ndo_fdb_add()")

ndo_fdb_add() parameter updated with compile time Linux kernel version
check.

Cc: stable@dpdk.org
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Tested-by: Bruce Richardson <bruce.richardson@intel.com>
Reviewed-by: Rami Rosen <ramirose@gmail.com>