perf test record.sh: Fix shellcheck warning

Add quotes to avoid the following warning:
```
In tests/shell/record.sh line 264:
 [ $(uname -m) = "s390x" ] && {
   ^---------^ SC2046 (warning): Quote this to prevent word splitting.

For more information:
 https://www.shellcheck.net/wiki/SC2046 -- Quote this to prevent word splitt...
```

Fixes: c73a56e ("perf test: Fix test case Leader sampling on s390")
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

Author: captain5050

tools/perf/tests/shell/record.sh

@@ -261,7 +261,7 @@ test_uid() {
 test_leader_sampling() {
   echo "Basic leader sampling test"
   events="{cycles,cycles}:Su"
-  [ $(uname -m) = "s390x" ] && {
+  [ "$(uname -m)" = "s390x" ] && {
     [ ! -d /sys/devices/cpum_sf ] && {
       echo "No CPUMF [Skipped record]"
       return