perf annotate: Store the sample period in each histogram bucket

We'll use it soon, when fixing --show-total-period.

Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
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: http://lkml.kernel.org/r/1500500215-16646-1-git-send-email-treeze.taeung@gmail.com
[ split from a larger patch, do the math in __symbol__inc_addr_samples() ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>