Tweak RefBase to build with 12.04 host gcc

Change-Id: I017a12ef379e93093ff5ff4aafab557b3632cd9f
diff --git a/libs/utils/RefBase.cpp b/libs/utils/RefBase.cpp
index 47ef546..545da7d 100644
--- a/libs/utils/RefBase.cpp
+++ b/libs/utils/RefBase.cpp
@@ -504,7 +504,7 @@
 
 void RefBase::weakref_type::trackMe(bool enable, bool retain)
 {
-    static_cast<const weakref_impl*>(this)->trackMe(enable, retain);
+    static_cast<weakref_impl*>(this)->trackMe(enable, retain);
 }
 
 RefBase::weakref_type* RefBase::createWeak(const void* id) const