perfcounters: include asm/perf_counter.h only if CONFIG_PERF_COUNTERS=y

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: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Tested-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>