perf list: Fix JSON segfault by setting the used skip_duplicate_pmus callback

Json output didn't set the skip_duplicate_pmus callback yielding a
segfault.

Fixes: cd4e1efb

 ("perf pmus: Skip duplicate PMUs and don't print list suffix by default")
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: James Clark <james.clark@arm.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20231129213428.2227448-2-irogers@google.com


[namhyung: updated subject line according to Arnaldo]
Signed-off-by: Namhyung Kim <namhyung@kernel.org>