perf hists browser: Save pstack in the hist_browser

perf hists browser: Save pstack in the hist_browser

The struct pstack is to save currently applied thread and/or dso filters
in the browser.  So it'd be better to keep in the struct hist_browser.
This is a preparation to later change.

Suggested-by: Arnaldo Carvalho de Melo <acme@kernel.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1429687101-4360-6-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>