perf stat: Add missing separator in the CSV header

It should have a comma after 'cpus' for socket and die aggregation mode.
The output of the following command shows the issue.

$ sudo perf stat -a --per-socket -x, --metric-only -I1 true

Before:

+--- here
V
time,socket,cpusGhz,insn per cycle,branch-misses of all branches,
0.000908461,S0,8,0.950,1.65,1.21,

After:

time,socket,cpus,GHz,insn per cycle,branch-misses of all branches,
0.000683094,S0,8,0.593,2.00,0.60,

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Xing Zhengjun <zhengjun.xing@linux.intel.com>
Link: https://lore.kernel.org/r/20221112032244.1077370-12-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>