kselftest: Add missing signature to the comments

The comment on top of the file is used by many developers to glance over
all the available functions. Add the recently added ksft_perror() to it.
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/kselftest.h

@@ -16,6 +16,7 @@
  * For each test, report any progress, debugging, etc with:
  *
  *     ksft_print_msg(fmt, ...);
+ *     ksft_perror(msg);
  *
  * and finally report the pass/fail/skip/xfail state of the test with one of:
  *