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 <[e-mail]>
Closes: https://lore.kernel.org/oe-kbuild-all/[e-mail]/
Suggested-by: Kees Cook <[e-mail]>
Signed-off-by: Amer Al Shanawany <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/seccomp/seccomp_benchmark.c

···
 	ksft_set_plan(7);
 
 	ksft_print_msg("Running on:\n");
-	ksft_print_msg("");
+	ksft_print_msg("%s", "");
 	system("uname -a");
 
 	ksft_print_msg("Current BPF sysctl settings:\n");
 	/* Avoid using "sysctl" which may not be installed. */
-	ksft_print_msg("");
+	ksft_print_msg("%s", "");
 	system("grep -H . /proc/sys/net/core/bpf_jit_enable");
-	ksft_print_msg("");
+	ksft_print_msg("%s", "");
 	system("grep -H . /proc/sys/net/core/bpf_jit_harden");
 
 	affinity();