perf_counter, x86: declare perf_max_counters only for CONFIG_PERF_COUNTERS

This is only needed for CONFIG_PERF_COUNTERS enabled.

[ Impact: cleanup ]

Signed-off-by: Robert Richter <robert.richter@amd.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <1241002046-8832-3-git-send-email-robert.richter@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>