perf tests stat_bpf_counters: Fix usage of '==' to address shellcheck warning

Running shellcheck on stat_bpf_counter.sh generates below
warning:

   In tests/shell/stat_bpf_counters.sh line 34:
   if [ "$base_cycles" == "<not" ]; then
                       ^-- SC3014 (warning): In POSIX sh, == in place of = is undefined.

   In tests/shell/stat_bpf_counters.sh line 39:
   if [ "$bpf_cycles" == "<not" ]; then
                      ^-- SC3014 (warning): In POSIX sh, == in place of = is undefined.

Fix this by using "=" instead of "==" to work for
all shells.
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/20230709182800.53002-10-atrajeev@linux.vnet.ibm.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/tests/shell/stat_bpf_counters.sh

@@ -31,12 +31,12 @@ if ! perf stat --bpf-counters true > /dev/null 2>&1; then
 fi
 
 base_cycles=$(perf stat --no-big-num -e cycles -- perf bench sched messaging -g 1 -l 100 -t 2>&1 | awk '/cycles/ {print $1}')
-if [ "$base_cycles" == "<not" ]; then
+if [ "$base_cycles" = "<not" ]; then
 	echo "Skipping: cycles event not counted"
 	exit 2
 fi
 bpf_cycles=$(perf stat --no-big-num --bpf-counters -e cycles -- perf bench sched messaging -g 1 -l 100 -t 2>&1 | awk '/cycles/ {print $1}')
-if [ "$bpf_cycles" == "<not" ]; then
+if [ "$bpf_cycles" = "<not" ]; then
 	echo "Failed: cycles not counted with --bpf-counters"
 	exit 1
 fi