perf symbols: Move split_kallsyms to struct map_groups

perf symbols: Move split_kallsyms to struct map_groups

Since it mainly will populate symtabs of its maps (kernel modules).

While looking at this I wonder if map_groups__split_kallsyms_for_kcore()
shouldn't be all that we need, seems much simpler.

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-3d1f3iby76popdr8ia9yimsc@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>