kni: disable FDB operations on RHEL 6.5

kni: disable FDB operations on RHEL 6.5

On RH 6.5:
igb_main.c:2298: error: unknown field ‘ndo_fdb_add’ specified in
initializer

FDB ops are present in RH 6.5 via the extension of netdev, so add the
ifdef inside the netdev ops definition of igb.

However, FDB functions are not set for RHEL 6.5: the implementation
relies on dev_mc_add_excl API which has not been backported.

Signed-off-by: Jean-Mickael Guerin <jean-mickael.guerin@6wind.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>