perf annotate: Introduce struct sym_hist_entry

struct sym_hist has addr[] but it should have not only number of samples
but also the sample period.  So use new struct symhist_entry to pave the
way to have that.

Committer notes:

This initial patch will only introduce the struct sym_hist_entry and use
only the nr_samples member, which makes the code clearer and paves the
way to save the period as well.

Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Suggested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1500500205-16553-1-git-send-email-treeze.taeung@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>