perf stat: Fix a segfault with --per-cluster --metric-only

The new --per-cluster option was added recently but it forgot to update
the aggr_header fields which are used for --metric-only option.  And it
resulted in a segfault due to NULL string in fputs().

Fixes: cbc917a1 ("perf stat: Support per-cluster aggregation")
Reviewed-by: Yicong Yang <yangyicong@hisilicon.com>
Tested-by: Yicong Yang <yangyicong@hisilicon.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Link: https://lore.kernel.org/r/20240628000604.1296808-1-namhyung@kernel.org


Signed-off-by: Namhyung Kim <namhyung@kernel.org>