[4.1.2-rt1 PATCH] include/linux/pid.h: fix build error

From: Grygorii Strashko <redacted>

This patch fixes build error:
   CC      kernel/pid_namespace.o
In file included from kernel/pid_namespace.c:11:0:
include/linux/pid.h: In function 'get_pid':
include/linux/pid.h:78:3: error: implicit declaration of function 'atomic_inc' [-Werror=implicit-function-declaration]
    atomic_inc(&pid->count);
    ^
which happens when
  CONFIG_PROVE_LOCKING=n
  CONFIG_DEBUG_SPINLOCK=n
  CONFIG_DEBUG_MUTEXES=n
  CONFIG_DEBUG_LOCK_ALLOC=n

Signed-off-by: Grygorii Strashko <redacted>
---
  include/linux/pid.h | 1 +
  1 file changed, 1 insertion(+)

diff --git a/include/linux/pid.h b/include/linux/pid.h
index 23705a53..2cc64b7 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h

@@ -2,6 +2,7 @@
  #define _LINUX_PID_H

  #include <linux/rcupdate.h>
+#include <linux/atomic.h>

  enum pid_type
  {