perf hists: Add _idx fields into struct perf_hpp_fmt

perf hists: Add _idx fields into struct perf_hpp_fmt

Currently there's no way of comparing hpp format entries, which is
needed in following patches.

Adding _idx fields into struct perf_hpp_fmt to recognize and be able to
compare hpp format entries.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1453109064-1026-4-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>