perf hists: Allocate output sort field

perf hists: Allocate output sort field

Currently we use static output fields, because we have single global
list of all sort/output fields.

We will add hists specific sort and output lists in following patches,
so we need all format entries to be dynamically allocated. Adding
support to allocate output sort field.

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/1453109064-1026-10-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>