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));
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
diff --git a/tests/loggingrb/loggingrb_test.c b/tests/loggingrb/loggingrb_test.c
index ebc0181..2e2c717 100644
--- a/tests/loggingrb/loggingrb_test.c
+++ b/tests/loggingrb/loggingrb_test.c
@@ -73,8 +73,8 @@
 	DEBUGP(DRLL, "You should see this\n");
 	DEBUGP(DCC, "You should see this\n");
 	DEBUGP(DMM, "You should not see this\n");
-	fprintf(stderr, log_target_rb_get(ringbuf_target, 0));
-	fprintf(stderr, log_target_rb_get(ringbuf_target, 1));
+	fprintf(stderr, "%s", log_target_rb_get(ringbuf_target, 0));
+	fprintf(stderr, "%s", log_target_rb_get(ringbuf_target, 1));
 	OSMO_ASSERT(!log_target_rb_get(ringbuf_target, 2));
 
 	return 0;