perf annotate: Split allocation of annotated_source struct

perf annotate: Split allocation of annotated_source struct

So that we can allocate just the notes->src->cyc_hist, that, unlike
notes->src->histograms, is not per event, and in paths where we
need to lazily allocate notes->src->cyc_hist we don't have the
number of events handy to also allocate ->histograms.

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-tsx7dhxzpi0criyx0sio3pz3@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>