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

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

Commit 3ce311afb558 ("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.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>