kni: fix build on CentOS 6.6

kni: fix build on CentOS 6.6

From CentOS 6.6, function skb_set_hash is introduced, this breaks
the previous assumption. So modify RHEL_RELEASE_VERSION from 7.0
to 6.6 to fix build for rte_kni.ko.

Related mail from Barak Enat:
http://dpdk.org/ml/archives/dev/2014-December/010124.html

building error likes:
  CC [M]  lib/librte_eal/linuxapp/kni/e1000_82575.o
In file included from lib/librte_eal/linuxapp/kni/ethtool/igb/e1000_osdep.h:41,
                 from lib/librte_eal/linuxapp/kni/ethtool/igb/e1000_hw.h:31,
                 from lib/librte_eal/linuxapp/kni/ethtool/igb/e1000_api.h:31,
                 from lib/librte_eal/linuxapp/kni/e1000_82575.c:38:
lib/librte_eal/linuxapp/kni/ethtool/igb/kcompat.h:3870: error: conflicting types for ‘skb_set_hash’
include/linux/skbuff.h:620: note: previous definition of ‘skb_set_hash’ was here

Reported-by: Barak Enat <barak@saguna.net>
Signed-off-by: Jincheng Miao <jincheng.miao@gmail.com>