MAINTAINERS: Add missing tools/lib/perf/ path to perf maintainers

Commit 3ce311af ("libperf: Move to tools/lib/perf") moved libperf
out of tools/perf/, but failed to update MAINTAINERS.
Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20200807193225.3904108-1-robh@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

MAINTAINERS

@@ -13554,6 +13554,7 @@ F:	arch/*/kernel/perf_event*.c
 F:	include/linux/perf_event.h
 F:	include/uapi/linux/perf_event.h
 F:	kernel/events/*
+F:	tools/lib/perf/
 F:	tools/perf/
 
 PERFORMANCE EVENTS SUBSYSTEM ARM64 PMU EVENTS