kni: fix build with kernel 4.11

kni: fix build with kernel 4.11

compile error:
.../build/build/lib/librte_eal/linuxapp/kni/kni_net.c:124:6:
error: implicit declaration of function ‘signal_pending’
[-Werror=implicit-function-declaration]
  if (signal_pending(current) || ret_val <= 0) {
      ^~~~~~~~~~~~~~

Linux 4.11 moves signal function declarations to its own header file:
Linux: 174cd4b1e5fb ("sched/headers: Prepare to move signal wakeup &
sigpending methods from <linux/sched.h> into <linux/sched/signal.h>")

Use new header file "linux/sched/signal.h" to fix the build error.

Cc: stable@dpdk.org
Reported-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Tested-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
Tested-by: Pankaj Gupta <pagupta@redhat.com>