selftests: seccomp: fix format-zero-length warnings

fix the following errors by using string format specifier and an empty
parameter:

seccomp_benchmark.c:197:24: warning: zero-length gnu_printf format
 string [-Wformat-zero-length]
  197 |         ksft_print_msg("");
      |                        ^~
seccomp_benchmark.c:202:24: warning: zero-length gnu_printf format
 string [-Wformat-zero-length]
  202 |         ksft_print_msg("");
      |                        ^~
seccomp_benchmark.c:204:24: warning: zero-length gnu_printf format
 string [-Wformat-zero-length]
  204 |         ksft_print_msg("");
      |                        ^~

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202312260235.Uj5ug8K9-lkp@intel.com/


Suggested-by: Kees Cook <kees@kernel.org>
Signed-off-by: Amer Al Shanawany <amer.shanawany@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>