MAINTAINERS: Add James Clark as a perf tools reviewer

MAINTAINERS: Add James Clark as a perf tools reviewer

James Clark has been actively reviewing patches and contributing
to perf tools. Reflect this by adding him as a reviewer in the
MAINTAINERS file.

Reviewed-by: James Clark <james.clark@linaro.org>
Acked-by: Ian Rogers <irogers@google.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>