sched/cputime: Fix IA64 build error of missing arch_vtime_task_switch() prototype

sched/cputime: Fix IA64 build error of missing arch_vtime_task_switch() prototype

The following commit:

  c89970202a11 ("cputime: remove cputime_to_nsecs fallback")

Removed an <asm/cputime.h> inclusion from <linux/sched/cputime.h>, but this
broke the IA64 build:

    arch/ia64/kernel/time.c:110:6: warning: no previous prototype for 'arch_vtime_task_switch' [-Wmissing-prototypes]

Add in the missing <asm/cputime.h> header to fix it.

Fixes: c89970202a11 ("cputime: remove cputime_to_nsecs fallback")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: linux-kernel@vger.kernel.org
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>