perf annotate: Move nr_events struct to 'struct annotated_source'

perf annotate: Move nr_events struct to 'struct annotated_source'

It's only used in 'perf annotate' output which means functions with actual
samples.  No need to consume memory for every symbol ('struct annotation').

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20240404175716.1225482-9-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>