perf annotate: Introduce annotated_source__alloc_histograms

So that we can call it independently, in contexts were we know we
already have notes->src allocated.

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-f5fn7tr1asey6g013wavpn4c@git.kernel.org


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