kni: fix build with kernel < 3.1

kni: fix build with kernel < 3.1

compile error:
  CC [M]  .../lib/librte_eal/linuxapp/kni/kni_misc.o
cc1: warnings being treated as errors
.../lib/librte_eal/linuxapp/kni/kni_misc.c: In function ‘kni_exit_net’:
.../lib/librte_eal/linuxapp/kni/kni_misc.c:113:18:
error: unused variable ‘knet’

For kernel versions < v3.1 mutex_destroy() is a macro and does nothing,
this cause an unused variable warning for knet which used in the
mutex_destroy()

mutex_destroy() converted into static inline function with commit:
Linux: 4582c0a4866e ("mutex: Make mutex_destroy() an inline function")

To fix the warning unused attribute added to the knet variable.

Fixes: 93a298b34e1b ("kni: support core id parameter in single threaded mode")

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