trusty: tipc_test: fix for building with -Wformat-security flag

Change-Id: I7aca7c7c8001ed82ed3503d622280bcd738da210
diff --git a/trusty/libtrusty/tipc-test/tipc_test.c b/trusty/libtrusty/tipc-test/tipc_test.c
index 7be2bb0..1fb34c9 100644
--- a/trusty/libtrusty/tipc-test/tipc_test.c
+++ b/trusty/libtrusty/tipc-test/tipc_test.c
@@ -96,7 +96,7 @@
 {
 	fprintf (stderr, usage, prog);
 	if (verbose)
-		fprintf (stderr, usage_long);
+		fprintf (stderr, "%s", usage_long);
 	exit(code);
 }