Use a class root for detecting java.lang.ref.Reference subclasses.

Change-Id: Ie2b74a5f2a16fedb49b65fbb67e13459877d8822
diff --git a/src/class_linker.h b/src/class_linker.h
index 67bb35c..a3157cc 100644
--- a/src/class_linker.h
+++ b/src/class_linker.h
@@ -351,6 +351,7 @@
     kClassArrayClass,
     kObjectArrayClass,
     kJavaLangString,
+    kJavaLangRefReference,
     kJavaLangReflectConstructor,
     kJavaLangReflectField,
     kJavaLangReflectMethod,