Syntax clean up to make clang happy.

Change-Id: I0984c395bbd1ee4b206eafd19915b6f68781dd16
diff --git a/src/thread.h b/src/thread.h
index efa92f4..499527a 100644
--- a/src/thread.h
+++ b/src/thread.h
@@ -46,7 +46,7 @@
 class ClassLinker;
 class ClassLoader;
 class Context;
-class DebugInvokeReq;
+struct DebugInvokeReq;
 class Method;
 class Monitor;
 class Object;