kni: fix build on SLES 12
kni: fix build on SLES 12

SLES 12 has kernel 3.12, which original does not have skb_set_hash,
but SuSE has added that function to the kernel integrated on it.
Therefore, the function is not declared when compiling on this OS.

Reported-by: Sotiris Salloumis <3fb4f9fe32462d07bfe8a379380a4ec3e23200e4@ericsson.com>
Signed-off-by: Pablo de Lara <28355d9cfe41bb8148aea0fb24f47aae8f4217a1@intel.com>

msune/dpdk,tsphillips/dpdk-fork,msune/dpdk,venkynv/dpdk-mirror,venkynv/dpdk-mirror,tsphillips/dpdk-fork,john-mcnamara-intel/dpdk,mixja/dpdk,john-mcnamara-intel/dpdk,tsphillips/dpdk-fork,tsphillips/dpdk-fork,venkynv/dpdk-mirror,msune/dpdk,mixja/dpdk,msune/dpdk,mixja/dpdk,john-mcnamara-intel/dpdk,venkynv/dpdk-mirror,john-mcnamara-intel/dpdk,mixja/dpdk