perf tests stat+csv_summary: Fix unused variable references detected via shellcheck

Running shellcheck on stat+csv_summary.sh throws below warnings:

   In tests/shell/stat+csv_summary.sh line 26:
   while read num event run pct
              ^-^ SC2034 (warning): num appears unused. Verify use (or export if used externally).
                  ^---^ SC2034 (warning): event appears unused. Verify use (or export if used externally).
                        ^-^ SC2034 (warning): run appears unused. Verify use (or export if used externally).
                            ^-^ SC2034 (warning): pct appears unused. Verify use (or export if used externally).

These variables are intentionally unused since they are needed to parse
through the output. Use "_" as a prefix for these throw away variables.

Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Disha Goel <disgoel@linux.vnet.ibm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: linuxppc-dev@lists.ozlabs.org
Link: https://lore.kernel.org/r/20230709182800.53002-8-atrajeev@linux.vnet.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>