Impact: build fix on ia64

KOSAKI Motohiro reported that -tip doesnt build on ia64 because
asm/perf_counter.h only exists on x86 for now. Fix it.
Reported-by: NKOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Tested-by: NKOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: NKOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>