perf annotate: Stop using symbol_conf.nr_events global in symbol__hists()

Since now we have evsel->evlist->nr_entries in the single place calling
this function, use it.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-9mgosbqa977h39j4i9ys8t75@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>