perf_counter: add MAINTAINERS entry

This adds an entry in MAINTAINERS for the perf_counter subsystem.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <18909.13033.345975.434902@cargo.ozlabs.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>