sched/rt: Add <linux/sched/rt.h> header to <linux/init_task.h>

IA64 relied on it through sched.h inclusion:

  arch/ia64/kernel/init_task.c:38:11: error: 'MAX_PRIO' undeclared here (not in a function)
  arch/ia64/kernel/init_task.c:38:11: error: 'RR_TIMESLICE' undeclared here (not in a function)

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Clark Williams <williams@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/n/tip-xaan1twswggedMR0airtpjui@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>