add virtual destructor to avoid compiler warning

panda/src/express/weakPointerCallback.cxx

@@ -17,3 +17,12 @@
 ////////////////////////////////////////////////////////////////////
 
 #include "weakPointerCallback.h"
+
+////////////////////////////////////////////////////////////////////
+//     Function: WeakPointerCallback::Destructor
+//       Access: Public, Virtual
+//  Description:
+////////////////////////////////////////////////////////////////////
+WeakPointerCallback::
+~WeakPointerCallback() {
+}

panda/src/express/weakPointerCallback.h

@@ -30,6 +30,7 @@
 ////////////////////////////////////////////////////////////////////
 class EXPCL_PANDAEXPRESS WeakPointerCallback {
 public:
+  virtual ~WeakPointerCallback();
   virtual void wp_callback(void *pointer)=0;
 };