perf stat: Making several helper functions static

perf stat: Making several helper functions static

There's no need for the following functions to be global:

  perf_evsel__reset_stat_priv
  perf_evsel__alloc_stat_priv
  perf_evsel__free_stat_priv
  perf_evsel__alloc_prev_raw_counts
  perf_evsel__free_prev_raw_counts
  perf_evsel__alloc_stats

They all ended up in util/stat.c, and they no longer need to be called
from outside this object.

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/1453290995-18485-5-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>