logginrb: Fix compiler warning about fprintf

loggingrb/loggingrb_test.c:76:18: warning: format string is not a string literal (potentially insecure) [-Wformat-security]
        fprintf(stderr, log_target_rb_get(ringbuf_target, 0));
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
loggingrb/loggingrb_test.c:77:18: warning: format string is not a string literal (potentially insecure) [-Wformat-security]
        fprintf(stderr, log_target_rb_get(ringbuf_target, 1));
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 file changed