Use correct WPARAM type for a MSW ID and not UINT

These types are of different size under Win64, so even if IDs are 32 bits,
truncating a 64 bit value to 32 bit UINT resulted in a warning. Fix this by
just using WPARAM for the variable type in the first place.

src/msw/menuitem.cpp

@@ -1403,7 +1403,7 @@ int wxMenuItem::MSGetMenuItemPos() const
     if ( !hMenu )
         return -1;
 
-    const UINT id = GetMSWId();
+    const WXWPARAM id = GetMSWId();
     const int menuItems = ::GetMenuItemCount(hMenu);
     for ( int i = 0; i < menuItems; i++ )
     {