perf hists: Introduce hists__for_each_format macro

With the hist object having the perf_hpp_list we can now iterate output
format entries based in the hists object. Adding hists__for_each_format
macro to do that.

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/1453109064-1026-26-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>