Fix TDM-GCC warning about int to pointer cast.

Use wxUIntToPtr() to suppress the warning, we know that the cast here is safe
because Windows uses only pointers fitting in the UINT range for the menu (and
other) handles.

src/msw/menuitem.cpp

@@ -1272,7 +1272,7 @@ int wxMenuItem::MSGetMenuItemPos() const
 
         if ( state & MF_POPUP )
         {
-            if ( ::GetSubMenu(hMenu, i) == (HMENU)id )
+            if ( ::GetSubMenu(hMenu, i) == (HMENU)wxUIntToPtr(id) )
                 return i;
         }
         else if ( !(state & MF_SEPARATOR) )