perf tools: Add Ian Rogers to MAINTAINERS as a reviewer

Ian has been reviewing perf tooling patches consistently for a long
time, so lets reflect that in the MAINTAINERS file so that contributors
add him to the CC list in patch submissions.

Reviewed-by: Ian Rogers <irogers@google.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>