kni: fix build on Oracle Linux 6.4 and RHEL 6.5

kni: fix build on Oracle Linux 6.4 and RHEL 6.5

The compile errors are copied as follows. The fixes came from
Linux drivers of ixgbe-3.21.2 and igb-5.1.2 with modifications.
The idea is to use self-defined functions no matter they have
already been defined somewhere or not.

* Oracle Linux6.4
lib/librte_eal/linuxapp/kni/ethtool/ixgbe/kcompat.h:3111:
error: redefinition of 'ether_addr_equal'
include/linux/etherdevice.h:180: note: previous definition
of 'ether_addr_equal' was here

* RHEL6.5
lib/librte_eal/linuxapp/kni/ethtool/igb/kcompat.h:3597:
error: redefinition of 'mmd_eee_cap_to_ethtool_sup_t'
include/linux/mdio.h:387: note: previous definition of
'mmd_eee_cap_to_ethtool_sup_t' was here
lib/librte_eal/linuxapp/kni/ethtool/igb/kcompat.h:3625:
error: redefinition of 'mmd_eee_adv_to_ethtool_adv_t'
include/linux/mdio.h:415: note: previous definition of
'mmd_eee_adv_to_ethtool_adv_t' was here
lib/librte_eal/linuxapp/kni/ethtool/igb/kcompat.h:3653:
error: redefinition of 'ethtool_adv_to_mmd_eee_adv_t'
include/linux/mdio.h:443: note: previous definition of
'ethtool_adv_to_mmd_eee_adv_t' was here

Signed-off-by: Helin Zhang <helin.zhang@intel.com>
Acked-by: Cunming Liang <cunming.liang@intel.com>
Tested-by: Waterman Cao <waterman.cao@intel.com>