kni: fix build with gcc 6.1

kni: fix build with gcc 6.1

Using gcc 6.1, in some cases, kni fails to compile
because of unused variables:

lib/librte_eal/linuxapp/kni/ixgbe_main.c:82:19:
error: ‘ixgbe_copyright’
defined but not used [-Werror=unused-const-variable=]

lib/librte_eal/linuxapp/kni/ixgbe_main.c:62:19:
error: ‘ixgbe_driver_string’
defined but not used [-Werror=unused-const-variable=]

Fixes: 3fc5ca2f6352 ("kni: initial import")

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>