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 <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>
Cc: [e-mail]
Cc: Nicholas Piggin <[e-mail]>
Cc: Peter Zijlstra (Intel) <[e-mail]>
Cc: Alexander Gordeev <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>

arch/ia64/kernel/time.c

···
 #include <linux/platform_device.h>
 #include <linux/sched/cputime.h>
 
+#include <asm/cputime.h>
 #include <asm/delay.h>
 #include <asm/efi.h>
 #include <asm/hw_irq.h>