perf tools: Move hist_entry__period_snprintf into stdio code

perf tools: Move hist_entry__period_snprintf into stdio code

Moving hist_entry__period_snprintf function into stdio code and making
it static, as it's no longer used anywhere else.

Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/n/tip-ah8ms343h8xygt20iqz91kz4@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>