Syntax clean up to make clang happy.

Change-Id: I0984c395bbd1ee4b206eafd19915b6f68781dd16
diff --git a/src/debugger.h b/src/debugger.h
index 4db9bd7..6e82001 100644
--- a/src/debugger.h
+++ b/src/debugger.h
@@ -31,7 +31,7 @@
 namespace art {
 
 struct AllocRecord;
-struct Thread;
+class Thread;
 
 /*
  * Invoke-during-breakpoint support.