Try to fix mac build.

Change-Id: I6c7faa1c80bc7c250d14859d51c910d50b7fb562
diff --git a/runtime/utils.cc b/runtime/utils.cc
index 9c94f6c..070d85e 100644
--- a/runtime/utils.cc
+++ b/runtime/utils.cc
@@ -1164,6 +1164,8 @@
     }
     os << "\n";
   }
+#else
+  UNUSED(os, tid, prefix, current_method);
 #endif
 }