perf cgroup: Remove misplaced __maybe_unused

perf cgroup: Remove misplaced __maybe_unused

The 'opt' parameter in parse_cgroups() _is_ used. The original patch
used '__used' that was even more confusing :-)

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: Stephane Eranian <eranian@google.com>
Cc: Wang Nan <wangnan0@huawei.com>
Fixes: 023695d96ee0 ("perf tool: Add cgroup support")
Link: https://lkml.kernel.org/n/tip-4jo2puz0empkoou6bbq460tl@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>