perf annotate: Move nr_events from annotate_browser to annotation struct

perf annotate: Move nr_events from annotate_browser to annotation struct

Paving the way to move more stuff out of TUI and into the generic
annotation library.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jin Yao <yao.jin@linux.intel.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-8vqax6wgfqohelot8j8zsfvs@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>