sched: Add "const" to is_idle_task() parameter

This patch fixes a build warning in -next due to a const pointer being
passed to is_idle_task().  Because is_idle_task() does not modify anything,
this commit adds the "const" to is_idle_task()'s argument declaration.
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

include/linux/sched.h

@@ -2074,7 +2074,7 @@ extern struct task_struct *idle_task(int cpu);
  * is_idle_task - is the specified task an idle task?
  * @tsk: the task in question.
  */
-static inline bool is_idle_task(struct task_struct *p)
+static inline bool is_idle_task(const struct task_struct *p)
 {
 	return p->pid == 0;
 }