Fix compilation failure for kernels above 3.13.11

skb_get_rxhash was renamed to skb_get_hash somewhere between 3.13.0
and 3.14.0. skb_get_rxhash does not exist in ubuntu sources for
3.13.0-32, for which the version code is 3.13.11. Make skb_get_rxhash
return skb_get_hash for versions above 3.13.11

Closes Bug:1359940

Change-Id: If3710bf55c0664a85cf8eb4b148f33bded653782