Fix c++11-narrowing error when using clang on Windows

Case value 0xfffffd9f results in the following error:
error: case value evaluates to 4294966687, which cannot be narrowed to type
'DISPID' (aka 'long') [-Wc++11-narrowing]

src/msw/mediactrl_am.cpp

@@ -1530,7 +1530,9 @@ void wxAMMediaBackend::Move(int WXUNUSED(x), int WXUNUSED(y),
 //---------------------------------------------------------------------------
 void wxAMMediaEvtHandler::OnActiveX(wxActiveXEvent& event)
 {
-    switch(event.GetDispatchId())
+    // cast to unsigned long to fix narrowing error with case 0xfffffd9f
+    // when using clang
+    switch (static_cast<unsigned long>(event.GetDispatchId()))
     {
     case 0x00000001: // statechange in IActiveMovie
     case 0x00000bc4: // playstatechange in IMediaPlayer