perf tools: Introduce hists__inc_nr_samples()

There're some duplicate code for counting number of samples.  Add
hists__inc_nr_samples() and reuse it.

Suggested-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1401335910-16832-2-git-send-email-namhyung@kernel.org


Signed-off-by: Jiri Olsa <jolsa@kernel.org>