Fix invalid cast in wxMSW AutoHANDLE::InvalidHandle()

Somehow this compiled with the previous gcc versions (as well as MSVS),
but a static_cast from an integer wxUIntPtr type to a pointer HANDLE
type is obviously invalid and a reinterpret_cast is needed here.

This fixes compilation with g++ 8.

include/wx/msw/private.h

@@ -145,7 +145,7 @@ protected:
     // implicitly convertible to HANDLE, which is a pointer.
     static HANDLE InvalidHandle()
     {
-        return static_cast<HANDLE>(INVALID_VALUE);
+        return reinterpret_cast<HANDLE>(INVALID_VALUE);
     }
 
     void DoClose()