ART: Fix Mac build

Fix Mac build after change-id Id2710d2fd44b7c3b3335973a9288979a5793638b.

Bug: 18933933
Change-Id: Ib0bb5082908d5dff33ef8f8a69d82c07406c176d
diff --git a/runtime/utils.cc b/runtime/utils.cc
index 7234ec0..dd0bdbb 100644
--- a/runtime/utils.cc
+++ b/runtime/utils.cc
@@ -1177,7 +1177,7 @@
     os << "\n";
   }
 #else
-  UNUSED(os, tid, prefix, current_method);
+  UNUSED(os, tid, prefix, current_method, ucontext_ptr);
 #endif
 }