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: 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>

include/linux/perf_counter.h

@@ -14,7 +14,10 @@
 #define _LINUX_PERF_COUNTER_H
 
 #include <asm/atomic.h>
-#include <asm/perf_counter.h>
+
+#ifdef CONFIG_PERF_COUNTERS
+# include <asm/perf_counter.h>
+#endif
 
 #include <linux/list.h>
 #include <linux/mutex.h>