kni: fix build with kernel < 3.0

kni: fix build with kernel < 3.0

Compile error:
  CC [M]  .../build/lib/librte_eal/linuxapp/kni/igb_main.o
.../build/lib/librte_eal/linuxapp/kni/igb_main.c:
In function ‘igb_check_swap_media’:
.../build/lib/librte_eal/linuxapp/kni/igb_main.c:1556:7:
error: variable ‘link’ set but not used [-Werror=unused-but-set-variable]
  bool link;
       ^

With Linux kernel >= v3.0 this warning disabled:
Linux: 8417da6f2128 ("kbuild: Fix passing -Wno-* options to gcc 4.4+")

Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>