perf tests stat+shadow_stat: Fix shellcheck warning about unused variable

perf tests stat+shadow_stat: Fix shellcheck warning about unused variable

Running shellcheck on stat+shadow_stat.sh generates below warning:

In tests/shell/stat+shadow_stat.sh line 48:
        while read cpu num evt hash ipc rest
                               ^--^ SC2034 (warning): hash appears unused. Verify use (or export if used externally).

This variable is intentionally unused since it is needed to parse
through the output. Use "_" as a prefix for this throw away variable.

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/20230709182800.53002-11-atrajeev@linux.vnet.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>