perf tests stat+std_output: Fix shellcheck warnings about word splitting/quoting

Running shellcheck on stat+csv_output.sh throws below
warning:

   In tests/shell/stat+csv_output.sh line 9:
   . $(dirname $0)/lib/stat_output.sh
     ^-----------^ SC2046 (warning): Quote this to prevent word splitting.

Fixed the warning by adding quotes to avoid word splitting.
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/20230709182800.53002-26-atrajeev@linux.vnet.ibm.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/tests/shell/stat+csv_output.sh

@@ -6,7 +6,7 @@
 
 set -e
 
-. $(dirname $0)/lib/stat_output.sh
+. "$(dirname $0)"/lib/stat_output.sh
 
 csv_sep=@