perf report: No need to have report_callchain_help as a global

It is used in a single place, move the declaration to that function.

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-p650ofrl8xike4dewxod51gg@git.kernel.org


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