perf map: Shorten map_groups__find_by_name() signature

perf map: Shorten map_groups__find_by_name() signature

Another step in the road to elliminate the MAP_{FUNCTION,VARIABLE}
separation, reducing the exposure to these details in the tools using
the symbol APIs.

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-8a1hvrqe3r5i0kw865u3uxwt@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>